


Mm. 




I:'l'!| 



•!] i :<>■,, 

=!t;:i? /(.■■•-Hi- ., 



ERRATA. 

Bottom of page 17 ; instead of "Ampere = turns" read ''Ampere —turns." 
Ques. 244, page 92 ; instead of" 0% " read " jfj " 

Rax Rb y. Re 



Ques. 264, page 106 ; instead of 
read 



Ra X Rl>-\-Rb X Rc-\- Rex Ra " 
RaX Rb X Re 



[Rax Rb)-\-{Rbx Rc)-\-{Re X Ra) " 
Ques. 517, page 200 ; instead of "precisely % ohm" read "about 244 ohms." 
Ques. 517, page 200 ; instead of " i ohm" read "450 ohms." 



THE 



Electrical Catechism 



533 PLAIN ANSWERS 
TO 53 3 PRACTICAL 
QUESTIONS ABOUT 
ELECTRICAL APPARATUS 



COMPILED FROM THE REGULAR ISSUES OF 



POWER 



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NEW YORK 

HILL PUBLISHING COMPANY 

WORLD BUILDING 
1902 




THE L.BRARY OF 
CONGRESS. 

Two Copies Received 

JAN 26 1903 

Copyright Entry 

CLASS a^ XXc. No 

% / I 1^ 

COPY B. 



Entered according to Act of Congress in the Year 1902 
by the Hill Publishing Company, in the ofiBce 
of the Librarian of Congress at Washington 



TH^t^ 



c • 6 • « e c 

• a e 
I e c c « t 



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\ 



PREFACE. 

In the preparation of the subject matter of this book it has 
been the aim of the author to discuss only such branches of elec- 
trical work as may be considered to come within the range of the 
average central station or isolated plant engineer. On this account 
the endeavor has not been to cover the entire electrical field, 
but rather to make the treatment of principles and practice in 
what might be termed the "heavier" branches of electrical engi- 
neering more exhaustive than is usual in a work of this kind. 

It goes without saying that no originality is claimed for the 
subject matter; didactic productions cannot be original in the strict 
literary sense; but a strong effort has been made to present the 
expository matter in logical sequence and in clear, every-day 
phraseology. 

Parts of the matter here presented have appeared in serial 
form in the pages of Power; such portions have been carefully 
revised to eliminate typographical errors and to bring them up to 
date wherever this might be required. 

The Author. 

New York, March 25, 1902. 



CONTENTS. 



CHAPTER I. 

First Mention of Electricity — Voltaic Cells — Flow of Current — The 
Ampere — Resistance — Ohm's Law — Battery Cells — Battery 
Connections — Series and Parallel Connections — Electrical 
Power — Electrical Units 

CHAPTER n. 
Magnetism — Strength of Magnets — Magnetic Attractions — Magnetic 
Reluctance of Iron — Permeability of Iron and Steel — Solenoids 
— Permanent Magnets — Steel for Permanent Magnets 

CHAPTER III. 

Principles of Dynamo and Motor Construction — Generation of 
E. M. F, — Armatures — Armature Calculations — Temperature 
of Windings — Multipolar Dynamo — Armature Winding of 
Multipolar Dynamos — Construction of Armature Core — 
Eddy Currents — Commutators — Brushes — Shunt Wind- 
ing — Rheostats — Series Field Winding — Compound Field 
Winding — Principle of Motor Operation — Function of the 
Commutator — Counter E. M. F. — Speed Variation of Shunt 
Motor — Motor Torque — Motor Horse- Power — Regulation — 
Motor Starters — Commutated Field Winding — Differential 
Field Winding — Reversal of Rotation — Controllers 

CHAPTER IV. 

Circuits and Wiring — Dynamos in Parallel — Three- Wire System — 
Series and Compound Dynamos — Voltage Drop — Constant 
Potential Circuits — Wire Sizes — Fuses — Classes of Wiring — 
Conduit Wire — Wiring Systems — Switches — Switchboard — 
Circuit Breakers — Switchboard Connections — Central Station 
Switchboard — Lightning Arresters 

CHAPTER V. 

Measuring Instruments and Measurements — Voltmeters — Recording 
Watt-Hour Meters — Galvanometer — Wheatstone Bridge — 
Joint Resistance of Parallel Circuits — Measuring Resistance 
by Drop 

CHAPTER VI. 

Alternating Currents — Current Reversals — Effective Electromotive 
Force — The Cycle 

CHAPTER VII. 

Alternating Current Generators — Revolving Field Alternator — In- 
ductor Alternator — Alternator Armature Winding — Two- 
Phase Armature Winding — Phase Difference — Three-Phase 
Alternator — Three-Phase Armature Connection 



CHAPTER VIII. 
Alternating Current Circuits — Alternator Field Excitation — Three- 
Phase Circuits — Combining E. M. F.'s Out of Phase — Alter- 
nator Field Excitation — The Rectifying Commutator — Lag of 
Current — Commutating a Lagging Current 

CHAPTER IX. 
Alternating Current Principles — Self-induction — Reactive E. M. F. — 
Impedance — Reactance — Impedance Diagrams — Phase Rela- 
tions — Apparent Watts — Angle of Lag — Power Factor — 
Mathematical Relations 

CHAPTER X. 
Transformers — Transformer Construction — Ratios of Transforma- 
tion — Transformer Core Magnetization — Size of Wire — Trans- 
former Windings — Transformer Connections — Step-Up and 
Step-Down Transformers — Transformers on Polyphase Cir- 
cuits 

CHAPTER XL 

Motor Circuits — Synchronous Motor — Speed of Synchronous Motor 
— Starting a Synchronous Motor — Polyphase Synchronous 
Motors — Induction Motor — Stator Windings — Rotor — Induc- 
tion Motor Torque — Two-Phase and Three-Phase Windings 
— Induction Motor Speed — Polyphase Windings 

CHAPTER XII. 

Rotary Converter — Armature Connections — Converter Regulation — 
Converter Voltages — Reactive Regulators 

CHAPTER XIII. 

Electric Light — Electric Arc — Life of Open-Arc Carbons — Life of 
Enclosed-Arc Carbons — Differential Clutch Mechanism — Feed- 
ing — Shunt Lamp Mechanism — Constant Potential Lamp — Au- 
tomatic Cut-Outs — Alternating Current Lamp — Alternating- 
Current Solenoid Cores — Regulating Series Alternating Cur- 
rent Circuits — Adjustable Reactance Coil — Series Circuit Reg- 
ulator — Constant-Current Transformer 

CHAPTER XIV. 

The Incandescent Lamp — Life and Efficiency — Filament Deteriora- 
tion — Effect of Abnormal Voltage 

CHAPTER XV. 
The Nernst Lamp 



ELECTRICAL CATECHISM. 



CHAPTER I. 



0. I — How long has the existence of electricity been known? 

A. — It is mentioned in a treatise on gems, written about 600 
B. C, by a Greek author named Theophrastus. 

Q. 2 — What statement does this writer make? 

A. — That a piece of amber when rubbed briskly with a piece of 
silk possesses the power of attracting to itself light bodies, such 
as dust or pieces of paper. 

Q. 3 — What has this to do with electricity? 

A. — When the amber manifests the power of attraction men- 
tioned, it is said to be electrified, or charged with electricity. 

O. 4 — What is the name given to this kind of electricity ? 

A. — Frictional or static electricity. 

Q. 5 — Why is it called static electricity ? 

A. — Because it is at rest as distinguished from active currents 
of electricity. 

Q. 6 — Can a body be charged with electricity except by fric- 
tion? 

A. — Yes ; by contact with a body already electrified. 

Q. 7 — How does electrification by contact take place? 

A. — Electricity is transferred from the body originally electri- 
fied to the body touched. 

Q. 8 — W^hat is this transfer of electricity called ? 

A. — It is called a discharge, with reference to the body origi- 
nally electrified. 

Q. 9 — What is meant by "active currents of electricity'' ? 

A. — The continuous discharge or passage of electricity, supplied 
to a body as fast as it can be taken away. Fig. i shows a pum.p. A, 
drawing water from a tank, B, and forcing it through a line of 
pipe, C, which discharges into the tank. The amount delivered 
into the tank must be evidently equal to the amount taken out by 
the pump. This is similar to the flow of a current of electricity. 

Q. 10 — How may a continuous current of electricity be pro- 
J' duced ? 



2 Voltaic Cells. 

A. — The most common means are voltaic cells, and dynamos. 

Q. II — What is a voltaic cell? 

A. — A glass jar containing some water having added to it a 
little sulphuric acid, together with two clean strips, one of zinc, Z, 
and one of copper, C, as in Fig. 2, forms a simple voltaic cell. 

Q. 12 — With the cell, as shown, will any current be produced? 




Puwer, S. Y. 



Fig.1 



A. — No. The cell provides the E.M.F. or pressure which causes 
electricity to flow, but in order to obtain an actual flow it is neces- 
sary to connect the two plates by means of a wire, as shown in 

Fig. 3- 

Q. 13 — What purpose does this wire serve? 

A. — It completes the path for the current to flow through. 

Q. 14 — What arrangement of a pump can be made to give 
similar results with water? 

A. — Place a pump in a tank of water, as in Fig. 4. If rotated. 





FIG. 2. 



Power, N. Y. 

FIG. 3. 



it will simply churn the water up. If, however, the piping shown 
in Fig. 5 is added, there will be a continuous flow of water 



Flozv of Current. 



through the pipe. This illustration is not absolutely correct, be- 
cause the return pipe need not touch the water in the tank, while 
the wire must touch both pieces, Z and C. These pieces are 
called electrodes, the zinc being the negative electrode and the 
copper the positive electrode. The wire connecting the two is 
called a conductor, because it "conducts" the current. 

Q. 15 — Will not any material conduct it? 

A. — No. Metals, acids and carbon are conductors of elec- 
tricity, and most other substances are non-conductors, or "insu- 
lators," practically. 

Q. 16 — Are insulators used in electrical work? 

A. — Yes; to prevent contact between a conductor and some 
outside metallic substance, which would cause the current to 
"leak" away from its conductor. 

Q. 17 — How does a current flow in a conductor? 

A. — From the positive to the negative electrode of the battery, 
or other source of electricity. 




' 


f 












^^e:^^ 


=3=-=^^^^^-^^r^^^^^^-r-^ 


l^=^=il 






B-3^3-^^^:-^ 


5^3£?£=^ 


--^^ 


r^ 


^oj 


fe- 











11 



FIG. 4. 



FIG. 



Q. 18 — In which direction does the current flow in the cell? 

A. — From the zinc (negative electrode) to the copper strip 
(positive electrode). 

Q. 19 — Will the cell furnish electricity indefinitely? 

A. — No ; the zinc strip will be observed to waste away. It is 
dissolved by the acid and parts with its latent energy while its 
atoms combine with the acid. This energy is expanded in forcing 
the electricity through the acid to the copper strip, through the 
copper strip and the wire back to the zinc strip. When the zinc 
is consumed no more current will be produced. 

O. 20 — Is an electric current visible? 



4 Flow of Current. ■ 

A. — No; its existence is shown by its effects. 

Q. 21 — What are some of the eft'ects produced by an electric 
current ? 

A. — A current flowing through a wire will heat it, and if the 
wire is thin and the current strong, this heating will be very ap- 
parent. If the current flows through water or other liquids it will 
decompose them. If the wire through which the current flows be 
led near a magnetic needle it will cause it to turn aside or deflect. 
If the current passes through the air across a break in a conductor 
a spark is visible across the gap. 

0. 22 — Describe an experiment that shows the deflection of a 
needle under the influence of a current? 

A. — Take a compass needle ; it will point north and south, as 
shown in Fig. 6. Now, hold a wire in which a current flows 
parallel to the direction of the needle. The needle will be de- 
flected so that it stands east and west, or at right angles to its 
previous position. 

O. 23 — In which direction will the needle be deflected? 

A. — If the current flows, as shown by the straight arrow in the 
figure, the pole marked, A^, will be twisted toward the east, as 
shown by the curved arrows. If the current were reversed, the 
pole, N, would deflect toward the west. If the current were from 
north to south, as shown, but the wire below the needle, the pole, 
N, would move toward the west. If the wire were below the 
needle and the current reversed, the needle would deflect toward 
the east. Table I. shows this : 



TABLE I. 

Position of 
Direction of Wire with 

Current. Regard 

to Needle. 



Pole N is 

Deflected 

Toward. 



North to South Above East 

North to South Below West 

South to North Above West 

South to North Below East 

Q. 24 — Describe an experiment that shows the effect of a cur- 
rent in decomposing a liquid through which it passes. 

A. — Carry two wires from the electrodes of a voltaic cell to a 
small cup containing dilute sulphuric acid, as in Fig. 7. Place 



Value of the Ampere. 5 

the free ends in the cup containing the solution, taking care that 
these ends do not touch each other. It will be noticed after a time 
that the end of one wire becomes covered with bubbles, which col- 
lect and rise to the surface, and that this is the wire connected to 




Power, N.T. 



FIG. 6. 



the zinc or negative electrode of the cell. The other wire be- 
comes cleaner and brighter, as if the acid were dissolving it. This 
is really the case, as will be proved by the sulphuric acid taking on 
a blue tinge. If the action be allowed to continue for a long 
enough time, the negative terminal will be found covered with a 
brown deposit, which is pure copper, and the solution in the small 
cup will be found to have changed from sulphuric acid to sulphate 
of copper. 

Q. 25 — What determines the amount of copper dissolved from 
one electrode and deposited on the other ? 

A. — The rate of flow of the current. 

Q. 26 — What is meant by the rate of flow ? 

A. — The strength of the current passing through a conductor. 
The unit of the rate of flow is called an ampere. 

Q. 2y. — What determines the value of the ampere ? 



H 







Power, N. Y. \^^^^E^r^ 



FIG. 7. 



A. — It is the unvarying rate which will deposit 0.001 118 of a 
gramme * of silver per second, when the current is forced through 



♦One ounce contains 28.3495 grammes, o.ooii 18 gramme 



20050 



ounce, scant. 



•6 Resistance. 

a solution of nitrate of silver, from one silver plate to another. 

Q. 28 — What is the ampere similar to in the flow of water? 

A. — To the cross-sectional area of a body of water flowing in 
a pipe. The volume of water (cubic inches or feet) is analogous 
to coulombs in electrical work. Amperes X seconds of time = 
coulombs. The symbol is Q, denoting quantity. 

Q. 29 — Does the wire or other conductor through which the 
current flows oppose the passage of the current? 

A. — Yes ; this opposition is called resistance. 

Q. 30 — Give an analogue to the resistance of conductors to the 
passage of a current of electricity. 

A. — When water flows in pipes or conduits it meets with a cer- 
tain resistance, which is known as friction. This corresponds 
roughly with the electrical resistance of conductors. 

Q. 31 — What determines the resistance of an electrical con- 
ductor ? 

A. — Its material, area and length. Different materials, even if 
all of one size, offer different resistances. If a material of certain 
area and length has a certain resistance, doubling its length will 
double its resistance. If the length remains constant, the larger 
the area, the smaller the resistance. 

Q. 32 — How does the area affect the resistance? 

A. — In somewhat the same way that the area of a pipe affects 
its resistance to the flow of water. If a wire of a certain area has 
a certain resistance per foot of length, a wire of twice the area 
will have half the resistance, because it is the same as two wires of 
the smaller size, side by side, giving two paths to the current in- 
stead of one. 

Q. 33 — Then the resistance per foot of a given material is less 
the greater its area? 

A.— Yes. 

Q. 34 — What units is resistance measured in? 

A. — Ohms. One ohm is the resistance of a column of mercury, 
one millimeter (0.03937 inch) square and 1.063 i^eters (41.85 
inches) high, at the temperature of melting ice (32 degrees Fahr.) 

Q. 35 — Why does current flow from a battery or a dynamo ? 

A. — The battery generates electrical pressure, or E.M.F. by the 
decomposition of its elements. A dynamo generates this pressure 



Ohm's Law. 7 

by induction, which will be explained later. This E.M.F. causes 
current to flow through a wire just as steam pressure forces steam 
through a pipe. 

Q. 36 — Is electrical pressure measured in pounds? 

A. — No ; the unit is the volt. 

Q. 37 — What fixes the value of a volt? 

A. — The other two units, ampere and ohm. One volt is that 
pressure which will cause a current of i ampere to flow through a 
resistance of i ohm. 

Q. 38 — Then there is no standard volt, like the standard pound 
used as the unit of pressure? 

A. — There is a scientific standard which need not be considered 
in practical work. The ampere and ohm have physical standards, 
as given above, and these determine the practical value of the 
volt. 

Q. 39 — What other names are given to electrical pressure ? 

A. — Electromotive force, difference of potential, tension and 
voltage ; electromotive force is generally written E.M.F., or £, 
as in Ohm's law, when it is expressed in symbols. 

Q. 40 — What is Ohm's law? 

A. — The current, expressed in amperes, that passes through a 
wire or other conductor, is equal to the E.A/E.F. in volts, that urges 
it on, divided by the resistance of the conductor, in ohms, that 
opposes the passage. v 

Q. 41 — How is this law expressed in symbols? 

E 

A.— c = r; 

R 
in which C = current in amperes, E = electromotive force in 
volts and R = resistance in ohms. 

(This notation is in general use, and will be employed through- 
out the rest of this catechism without further explanation.) 

Q. 42 — Deduce the value of E from the equation given under 
Question 41. 

A.— E = CR, 

or, the E.M.F. is equal to the product of the current and the 
resistance. 

Q. 43 — And how is the value of R deduced ? 



Connecting Battery Cells. 



A.— 



R 



E 



or, the resistance is equal to the E.M.F. divided by the current. 

Q. 44 — How much E.M.F. will a battery like that described 
under Question 24 generate? 

A. — Such a battery is impractically weak. The commercial 
battery cells give an E.M.F. of a little less than i volt. 




+ I MAIN 



FIG. 8. 



Q. 45 — How can higher E.M.F. be obtained? 

A. — Higher E.M.F. can be obtained by connecting a number 
of batteries ''in series," as shown in Fig. 8. Then the E.M'.F. of 
each cell or jar is added to that of the other cells. If each of the 
cells in Fig. 8 has an E.M.F. of f volt, the E.M.F. at the ends 
marked + Main and — Main will be four times that value, or 
3 volts. 

Q. 46 — The illustration shows the positive electrode of one jar 
connected to the negative of the next one all the way along. Is 
this intentional? 

A. — Yes ; the positive electrode of a cell must always be con- 
nected to the negative of its neighbor when the cells are in series, 
because the current must flow from positive to negative outside of 



+ 



-o- 



2 

■o 



a 

o 



7 



-O^::^. 



FIG. 9. 



a cell, and from negative to positive inside, and if two cells of 
equal strength were connected, zinc to zinc, or copper to copper, 
they would oppose each other and no current would flow when the 
outer ends were connected by a wire. 



Connecting Battery Cells. 9 

Q. 47 — ''Connected in series" means in a row, then, one after 
the other? 

A. — Yes. Electrical connections are effected by mere contact, 
and when several devices, similar or dissimilar, are arranged in 
contact with each other so that the whole current passes through 
all of them, as in Figs. 8 and 9, they are said to be connected in 
series. 

Q. 48 — What other methods of connection are used? 




FIG. 10. 

A. — Parallel, or multiple connection, as in Figs. 10 and 11, and 
combinations of this and the series connection. 

Q. 49 — What is the object in connecting up in parallel? 

A. — Batteries and dynamos are connected in parallel when the 
E.M.F. of one is sufficient, but its current capacity is not. Adding 
other batteries or dynamos in parallel with one already in use does 
not increase the available E.M.F. , but gives greater current ca- 
pacity, just as connecting two or more boilers to one steam main 
gives more steam supply, but no greater pressure. If the output 

-{-MAIN 



— OR RETURN MAIN 

FIG. 11. 



of one cell of battery were f 



volt and i 



ampere, the group in 
Fig. 8 would supply 1 ampere at f X 4 = volts and the group 
in Fig. 10 would yield ^ X 3 = 2 ampere at f volt. 

Q. 50 — What combinations of series and parallel grouping are 
used? 



lO 



Complex Connections. 



A. — Series-parallel, as shown in Fig. 12, and parallel-series, as 
in Fig. 13. 

Q. 51 — What is the difference between the result obtained by 
the Fig. 12 arrangement and that obtained in Fig. 13 ? 

A. — None ; in both cases the E.M.F. is 0.75 X 3 = 0.225 volt 
and the current capacity is 0.1666 X 3 = i ampere, reckoning on 
the basis of f volt and 1/6 ampere per cell. 

O. 52 — Why are both methods used ? 

A. — Series-parallel connection is used when the current passing 
through the group is constant and the E.M.F. at the main ter- 
minals is variable. Parallel-series connection is used when the 
E.M.F. at the terminals is constant and the quantity of current 

TERMINAL 

I A 




\^a 




TERMINAL B 
FIG. 12. 




FIG. 13. 



is variable. Thus, in Fig. 12, it would be troublesome to cut out 
the cells i, 4 and 7 in order to reduce the current capacity, the 
severing of three distinct connections being necessary; on the 
other hand, the E.M.F. of the group may be easily reduced by 
transferring the main terminal. A, to the wire, a, involving a sin- 
gle operation and cutting out the cells, i, 2, 3. In Fig. 13, on 
the contrary, the current capacity is reduced by simply discon- 
necting the wire, h, and throwing out cells i, 4 and 7, but it would 
be troublesome to vary the E.M.F. by cutting out and in the cells, 
I, 2 and 3. 



Rules for Series and Parallel Connections. 



II 



Q. 53 — Are other devices than batteries connected in series- 
parallel and parallel-series? 

A. — Yes ; incandescent lamps are sometimes so connected, and 
magnet coils are frequently combined in such groups. Figs. 14 
and 15 show groups of devices arranged to correspond with the 
battery groups in Figs. 12 and 13. 



o 



o 



o 



FIG. 14. 

Q. 54 — Is there any rule applying to the connection of electrical 
devices ? 

A. — No specific rule is needed. As a guide, for use until the 
underlying principles become thoroughly familiar, the following 
will serve : 

I. — Connecting devices in series adds their individual E.M.F's 
and resistances; the current capacity of a series group is limited 
to the capacity of the weakest member. 

II. — Connecting devices in parallel add their individual current 
capacities. The E.M.F. of parallel sources of current is that of 
the strongest member; they should be equal. The allowable volt- 
age of a group of receptive devices in parallel is limited to that 
of the weakest member; all should be equal. The total resistance 

+ MAIN 



Power, J.V. ¥. 



FIG. 15. 



of a parallel group of devices of equal resistances is the resistance 
of one device divided by the number of them. 

III. — In series-parallel groups each sub-group of devices in 
parallel may be considered a unit member with reference to the 
entire group. 



12 



Electrical Power. 



IV. — In parallel-series groups each sub-group of devices in 
series may be considered a unit member with reference to the en- 
tire group. 

Q. 55 — In Fig. II, why does not all the current pass through 
the nearest lamp, marked i ? 

A. — Because its resistance is too high. The current that passes 
through each lamp of the group follows Ohm's law (page 7), 
and therefore is equal to the potential difference, E, between the 
mains, measured at the ends of the cross wires (called "taps"), 
divided by the resistance, R, of each tap and lamp from main to 
main. A hydraulic analogue is represented by Fig. 16, where P 
is a pump, taking water from a tank, R, and forcing it through 
six water-motors. The pipes, A and B, correspond with the mains 
of a parallel circuit (Fig. 11), and the little pipes connecting the 
motors with the mains correspond with the taps. 

Q. 56 — How is electrical power calculated ? 




Puwer, -V. r. 



FIG. 16. 



A. — In watts, the unit being named for James Watt. One watt 
is I ampere flowing under E.M.F. of i volt. Multiplying the 
amperes flowing in a circuit by the volts at the terminals gives the 
watts that are being applied to that circuit, or the rate of elec- 
trical working. 

Q. 57 — How does a watt compare with a mechanical horse- 
power ?" 

A. — One horse-power equals 746 watts.* 



This equivalent is always used in practice. The actual value is 745.956 watts. 



Electrical Units. 13 

Q. 58 — Is there an electrical unit corresponding to the me- 
chanical foot-pound? 

A. — Yes, in function but not exactly in value. The joule is the 
electrical unit of work, and is equal to i watt for one second of 
time. One watt for one minute, therefore, equals 60 joules, and 
10 watts for one minute = 600 joules. 

Q. 59 — What is the relation between joules and foot-pounds? 

A. — One joule equals 1.35625 foot-pounds; or i foot-pound 
equals 0.7373 joule. 

Q. 60 — Are there other electrical units? 

A. — Yes ; derived units, such as watt-hour, meaning i watt one 
hour; kilowatt, meaning 1000 watts; kilojoule, meaning 1000 
joules; kilowatt-hour, etc. 

Q. 61 — Why are so many units necessary? 

A. — They are not necessary — merely convenient. It is easier to 
say and write 10 kilowatts, than 10,000 watts. Moreover, the 
physical conception is easier, as a kilowatt is nearer a horse-power 
in value, and we are accustomed to reckoning in horse-powers. 

Q. 62 — Are these units represented by symbol letters ? 

A. — Most of them are. Watts are usually represented by W, 
and kilowatt is universally written KW. Joules are generally 
represented by /. Using these symbols, and representing seconds 
of time by t, and horse-power by HP, the relations of the various 
principal units may be summarized for convenient reference as 
below : 

W=CXR = §'=CXE ( I ) 

W =z y^e X HP (2) 

W = J-^t = QXE^t (3) 

1-3563 XFt.Lhs. Ft.Lbs. 
W = = (4) 

i 0-7373 X t 

E" X t 

J=WXt = CXRXt^ QXE (s) 

R 
J==:746XtXHP (6) 

^ — 1-3563 X Ft.Lbs. = Ft.Lbs. -^ 0.7373 (7) 

Ft.Lbs. = 0.7373 X / = 0.7373 XQXE (8) 

Ft.Lbs. z=ssoXtX HP (9) 



14 Electrical Units. 

tX W 
Ft.Lbs = 07373 XtXW = (10) 

1.3563 
HP = ^-^ 746 = 0.00134 XW (11) 

Ft.Lbs. 
HP = (12) 

550 X^ 

/ QXE 

HP^ = (13) 

746 X ^ 746 X ^ 

HP = KW ~ 0.746 = KW X 1.34* (14) 

KW = 0.746 X HP = HP -^ 1.34* (15) 

1356.3 X Ft.Lbs. 

KW = (16) 

t 

* Approximate. More accurately, 1.34056. 



CHAPTER IL 
MAGNETISM. 

Q. 63 — What Is a magnet? 

A. — A magnet is a bar of iron or steel which possesses the 
power of attracting iron, steel, and in a slight degree, nickel. 

Q. 64 — What is this power called, and how else is it mani- 
fested ? 

A. — Magnetism; its presence may be clearly shown by sprink- 
ling iron filings on a sheet of paper under which a magnet is held. 
If the magnet is a straight bar the filings will arrange themselves 
as shown in Fig. 17. The distribution of the filings indicates that 
magnetism consists of invisible lines of force, and it is conse- 




FIG. 17. 

quently regarded and spoken of as "lines of force," and ''magnetic 
lines." 

Q. 65 — What other term is used to describe magnetism? 

A. — Magnetic flux ; and where the flux passes from pole to 
pole of a magnet through air or other non-magnetic material it is 
called the magnetic field. 

Q. 66 — Is non-magnetic material an insulator for magnetism? 

A. — No. There is no magnetic insulator. All materials except 
iron, steel and nickel, are poor magnetic conductors, of practically 
equal magnetic resistance ; iron and steel are good conductors of 
magnetism, hence they are called magnetic material. Magnetiza- 
ble material would be a more correct term, but it is cumbersome. 

Q. 67 — In which direction do magnetic lines flow ? 



i6 



Electromagnets. 



A. — From the north to the south pole''' of a magnet, outside of 
the magnet, and from the south to the north pole within the mag- 
net, as shown in Fig. i8. 

Q. 68 — What are the ''poles" of a magnet ? 

A. — The ends from, and to which, the magnetic flux passes. 

Q. 69 — What is an electro-magnet ? 

A. — If a wire be wound around a bar of soft iron or steel, as in 
Fig. 19, and a current sent through the wire, the bar will be found 
to be magnetized as long as the current is passing. If the bar is of 




'%'mf 



Power, N.T. 

FIG. 18. 



soft iron it will be very strongly magnetized, but will not retain 
its magnetism for any length of time after the current ceases to 
flow. If the bar is of steel, it will not be magnetized so strongly 
nor so quickly, but it will retain its magnetism for a greater length 
of time after the current is shut off. Such an arrangement is an 
electro-magnet.- 

Q. 70 — Does it make any difiference how the wire is wound 
around the bar? 

A.— Yes ; it should be wound continuously in one (either) di- 
rection. Then the polarity, or location of the poles, of the bar 
depends on the way the current flows through the wire. 

Q. 71 — How may the polarity of a magnet be determined? 

A. — If the current passes around the magnet clockwise, as in 



* More accurately, the north-seeking and south- seekins 
compass needle is the one which points north. 



poles. The north pole of a 



Strength of a Magnet. 



17 



Fig. 19, the magnetic flux in the magnet will be away from the 
observed end of it. For example, if current passes around the 
hand spindles of a clock in the direction its hands travel, the end 
of the spindle on which the hands are mounted will be the south 




FIG. 19. 

pole. The poles may be identified by holding a permanent mag- 
net, or a compass needle, near one pole of the electro-magnet ; the 
north pole of one will attract the south pole of the other, and vice 
versa. 

Q. y2 — What governs the strength of a magnet? 

A. — The number of ampere-turns* in the magnetizing coil and 
the quality of the path through which the flux passes. Magnetic 
lines form closed loops, as indicated by Figs. 20 and 21, and the 
strength of the magnetic flux is measured in lines per square inch 




rt 




/ i 



N 



Power, N. Y. 



,> / 



FIG. 20. 



FI(i. 21. 



area of path. This number is greater the greater the exciting 
force in ampere-turns, or the better the magnetic quality of the 
pathway. 

Q. 73 — What determines the "quality of the pathway?" 

A. — The material. Annealed wrought iron is the best ; annealed 



* Ampere = turns = Amperes of current passing through a coil X number of turns 
of wire in the coil. 



i8 



Maonetic Attractioii. 



cast steel comes next and soft cast iron third. Air and metals 
other than iron and steel are the poorest. 

O. 74 — Why does a magnet draw other pieces of iron to it ? 

A. — Because (i) the iron is a better conductor of magnetism 
than air and (2) lines of force exert their powers in the direction 
of shortening their travel. Hence, a piece of iron placed within 
range of the flux from a magnet and left free to move, as in Fig. 
20, will be pulled into that position which gives the lines of force 





SIDE VIEW 



Power, N. 




FIG. 23. 



FIG. 22. 



the shortest path through it from the north to the south pole of the 
magnet — nameh', in contact with both poles, as in Fig. 21. 

Q. 75 — Are magnets used to operate mechanism by their power 
of attraction? 

A. — Yes ; in numerous devices, of which some of the most 
familiar are electric bells, electric arc lamps, automatic electric 
regulators, electric locks and telegraph instruments. 

Q. y6 — What is the name of the movable piece upon which the 
magnet exerts its pull ? 

A. — The armature. Fig. 22 shows a typical arrangement of a 
magnet with an armature pivoted at one end. Fig. 23 shows one 
with the armature pivoted in the center. In both and any cases, 



Magnetic Reluctance of Iron. 



19 



the armature will always be drawn into that position which gives 
the magnetic lines of force the shortest path from pole to pole 
through the armature, within the range of its movement. 

Q. 77 — Is there any limit to the magnetic flux that can be forced 
through iron and steel? 

Table II 



Density of 
Magnetization. 




Permeability. 




Lines Per Square 


Annealed 


Commercial 


1 
Gray 1 Ord 


inary 


Inch B° 


Wrought Iron. 


Wroug'ht Iron. 


Cast Iron. ' Cast Iron. 


20 000 


2,600 


1,800 


850 650 


25.000 


2,9<i0 


2.000 , 800 700 


30.000 


3.000 


2,100 600 7 


70 


35,000 


2.950 


2.150 


400 ^ 


500 


40 000 


2.900 


2,130 


250 7 


70 


45 000 


2.800 


2,100 


140 7 


30 


50 000 


2.650 


2,050 


110 7 


'00 


55,000 


2,500 


1,980 


90 ( 


500 


60,000 


2,300 


1850 


70 I 


)00 


65 000 


2.100 


1,700 


50 ^ 


150 


70.000 


1.800 


1,550 


35 i 


^50 


75.000 


1,500 


1.400 


25 i 


J50 


80.000 


1,200 


1,250 


20 5 


>00 


85,000 


1,000 


1,100 


15 ] 


50 


90,000 


800 


900 


12 ] 


LOO 


95.000 


530 


680 


10 


70 


100.000 


360 


500 


9 


50 


105.000 


260 


36) 






110.000 


180 


260 










115 000 


120 


190 










120.000 


80 


150 










125 000 


50 


120 










130.000 


30 


100 








135.000 


20 


85 


. 








140,000 


15 


75 




1 







A. — Yes ; beyond certain degrees of magnetization, called 
"working points," the magnetic resistance of iron increases so 
rapidly that a considerable increase in magnetizing power pro- 
duces only a small increase in magnetism. Then the magnet core 
is said to be nearing "saturation." Finally a point is reached when 
an increase in magnetizing power produces no appreciable in- 
crease in magnetism; then the core is saturated.* Table II. 
gives the permeability of various grades of iron and steel at dif- 



* When a cloth is so wet that further application of water does not add to its mois- 
ture, it is " saturated." Similarly, when a piece of iron does not appreciably increase 
in ma}?netic strength under an increase in magnetizing force, it is called '' saturated." 



zo Permeability of Iron and Steel. 

ferent degrees of magnetization. Permeability is the ability to 
conduct magnetism or to contain magnetic lines. It corresponds 
to the electrical conductivity of wires. 

Q. 78 — Is there a name for magnetic resistance? 

A. — Yes ; reluctance. The law of the magnetic circuit is 
analogous to Ohm's law for electrical circuits ; namely, 

Alasrnetizin^ force 

Flux ^ ^ ^ 

Reluctance 

But the calculation of a magnetic circuit is much more difficult 
than that of an electric circuit, because of the leakage of mag- 
netic lines ; there being no such thing as an insulator of mag- 
netism. If we represent ampere-turns by M, the permeability of 
the material by ^, the area of the magnetic pathway by a, its 
length by /, and the total number of magnetic lines by ^ , the 
formula in symbol is : 

if leakage is neglected. In many cases it is more convenient to 
reckon in magnetic ''density" instead of total flux; if the mag- 
netic density, in lines of force per square inch of cross-section is 
represented by B we can use the formulas 

B= ^^ , (18) 

0.3133 x/ ^ 

and ^ = ^-3^33 X/^ 

fJL V i'/ 

for iron and steel. Where the lines pass through air the per- 
meability, /Lt , is z , and may be omitted. 

Q. 79^How is the permeability of iron and steel obtained? 

A. — The permeability is equal to the value of <l> when iron is 
the magnetic conductor of the flux, divided by the value of <l> when 
air is the conductor. Thus, if we pass current through a coil of 
wire, as in Fig. 24, even though there is no iron core present, 
magnetic lines will be created, and form closed circuits, as indi- 
cated by the dotted lines. Now, suppose we measure the flux pass- 
ing through the hollow of the coil and find 1000 lines. Then if 
we insert an iron core and connect its ends with a yoke so as to 
give a complete circuit of iron, as in Fig. 25, and find that the 



Solenoids. 



21 



number of lines through the coil has risen to 500,000, with the 
same number of ampere-turns, the "permeability" of the iron core 

500^000 

will be — = 500. 

1000 

Q. 80 — How are the lines of force in the coil and in the iron 
core measured? 

A. — There are several methods of measuring them, all of which 
are of a scientific nature and need not be discussed here. The in- 
strument usually used in measuring the lines is called a ballistic 





Power, A', y. 



Power, N. T. 



FIG. 24. 



FIG. 25. 



galvanometer. Its use is described in more advanced text books. 

Q. 81 — What is the unit of permeability ? 

A. — There is no unit ; it is simply the ratio between the fiux in 
iron or steel and that in air, with a given magnetizing force. 

0. 82 — If magnetism is created by a coil alone, as in Fig. 24, 
is not the coil a magnet ? 

A. — Yes, literally. A distinction is made in practice, however, 
between a coil alone and a coil on a core. The coil alone is called 
a solenoid. 

Q. 83 — Are solenoids used in electrical apparatus ? 

A. — Yes ; for many purposes, as in arc lamps and regulators. 

Q. 84 — Why is it not better to put a core m the coil, if it in- 
creases the magnetism so greatly ? 



22 



Solenoids. 



A. — It is, when the magnetic attraction is exerted over very 
short distances. But where the armature or moving member 
must have a 'long range of action without excessive variation in 
the ''pull" exerted upon it by the coil a solenoid is preferable, 
though weaker than a magnet. 

Q. 85 — How is a solenoid used ? 

A. — In connection with an iron plunger, as shown by Fig. 26. 
This plunger takes the place of the armature of a magnet, and 
its movement is due to the same cause ; namely, the effort of mag- 
netic lines of force to shorten the length of their path and improve 
its quality. 



XjULSii^--"^ 




FIG. 26. 



O. 86 — Why do the lines of force pull a plunger further than 
an armature? 

A. — Because the plunger has to move a greater distance from 
the point where it is just within range to the point where it gives 
the lines their shortest path through it. Figs. 2"/ and 28 give the 
comparison. In Fig. 27 the lines have to reach out the same dis- 
tance from the solenoid to the plunger that they do from the mag- 
net core to the armature ; in Fig. 28 the plunger has moved many 
times the distance traversed by the armature in order to reach the 
"shortest path" position. 

Q. 87 — Is not a solenoid more efficient than a magnet then, on 
account of its long pulling range ? 

A. — No ; because the strength of the pull is much less than that 
of a magnet. 



Permanent Magnets. 



23 



Q. 88— What limits the range of pull of a solenoid? 

A.— Its own length. At the start the plunger end should be just 
at the end of the coil or a trifle within it ; the plunger will be drawn 
through until its ends project equal distances beyond the ends of 
the coil, as in Fig. 28. The pull at the start is comparatively weak, 
gradually increasing as the plunger enters and falling off again 
as its end approaches the distant end of the coil. There is a long 
range, however, of strong pull within the coil. 

Q. 89 — In Fig. 17, 18, 20 and 21, the magnets shown have no 
coils of wire on them. Why is this ? 

A. — They are permanent magnets; Fig. 17 is a "bar" magnet 
and the others are "horse-shoe" magnets. 

jn 




COIL 



PLUNGER 





PLUNGER 



FIG. 27. 



FIG. 28. 



Q. 90 — How does a permanent magnet differ from an electro- 
magnet ? 

A. — When once magnetized, it remains so indefinitely. Such a 
magnet is made of hard steel. Soft iron or steel will not retain 
magnetism an appreciable length of time after the magnetizing 
power is withdrawn, and for that reason it is preferable for elec- 
tro-magnets whose strength is required to respond to variations 
in the magnetizing power of their coils. 

Q. 91 — What are permanent magnets used for? 

A. — To furnish the "magnetic field" in measuring instruments, 
such as voltmeters and ammeters, and in the familiar magneto- 



24 



Steel for Permanent Magnets. 



electric machines used in some telephone systems and for testing 
purposes. 

Q. 92 — How is a piece of steel made into a permanent magnet ? 

A. — Either by winding a coil on it and sending a powerful cur- 
rent through the coil, or by rubbing it on the poles of a powerful 
electro-magnet. 

Q. 93 — What is the best material for permanent magnets? 

A. — Tungsten steel or chrome steel. The analyses of tungsten 
and chrome steels are as follows : 



TUNGSTEN STEEL. 

Iron 

Carbon 

Manganese 

Silicon 

Phosphorus 

Tungsten* *. 3 



CHROME STEEL. 

95.371 Iron .- 

511 Carbon 

625 Manganese 

021 Sulphur 

028 Silicon 

444 Phosphorus 

Chromium* 



97-893 
.687 
028 
020 

134 
043 
195 



Tungsten and chromium are minerals. 



CHAPTER m. 

PRINCIPLES OF DYNAMO AND MOTOR 
CONSTRUCTION* 

Q. 94 — Under Question 35 it was stated that a dynamo gen- 
erates E.M.F. by induction; what is induction? 

A. — The induction there mentioned is magneto-electric* induc- 
tion, or the inducing of electricity by magnetism. The creation 
of magnetic lines of force by means of a current of electricity is 
electro-magnetic* induction. (See Q. 69.) 

Q. 95 — How is magneto-electric induction obtained? 

A. — Either by moving a conductor (such as a wire) through 
a stationary magnetic field at right angles to the flux, as in Fig. 




FIG. 29. 



Power, X. r. 



29, or by moving the magnetic field so as to cause the lines to be 
cut by a stationary conductor. 

Q. 96 — How much E.M.F. is generated by moving a wire 
across lines of force? 

A. — One volt for each 100,000,000 lines passed per second by 
the wire. This moving across lines of force is called ''cutting." If 
100,000,000 lines of force are passing from A^ to S, Fig. 29, and 
the wire were carried downward at such a rate of speed that it 
moved from one edge of the magnetic field to the other in one 
second, it would "cut" 100,000,000 lines per second, and i volt 
E.M.F. would be generated in it. 



* Magneto-electric, meaning, roughly, magnetism converted into electricity. Electro- 
magnetic, meaning, roughly, electricity converted into magnetism. 



26 Generation of E.M.F. 

Q. 97 — Then it would only be necessary to move the wire back 
and forth across the magnetic field in order to keep generating- 
E.M.F. ? 

A. — Yes ; but when it is moved upward the E.M.F. is reversed 
in direction, so that if it were simply moved back and forth, and 
its ends were connected by a wire outside of the field, the current, 
in the closed circuit would flow first in one direction and then in 
the other ; this sort of current is called "alternating." 

Q. 98 — How is the wire moved across the magnetic field in a 
dynamo ? 

A. — Many wires are used instead of one, and these are laid 
along a cylindrical iron structure, called the armature core, which 
is revolved so as to carry the wires across the field many times 
per second, but in a curved path instead of a straight fine. Refer- 
ence to Fig. 30 will make this clear. The lines of force pass from 




FIG. 30. 

A^ to the iron cylinder and from that to S; revolving the cylmder 
will cause the wires wound on the cylinder to cut across the lines 
of force, and an E.M.F. will be generated in the wires. If the 
ends of the coil are connected together or to an outside conductor, 
current will flow. 

Q. 99 — But half of the wires move upward, while the other half 
move downward. Does not this interfere? 

A. — No ; although the half,. A, is cutting lines in one direction 
when the half, B, is cutting in the opposite direction, the flow 
through all parts of the coil agrees. Thus, when A passes down- 
ward between vS and the core, the current tends to flow from the 
front end of the core to the back in A; as B is passing upward 
between A^" and the core, the current in B tends to flow from back 
to front of the core, joining its effort, through the wires across 
the ends, with that in A. 



Armatures. 



27 



Q. 100 — But when A comes around to N' and moves upward, 
is not the E.M.F. reversed, as it was in the single wire? 

A. — Yes. In order to prevent the reversal of the E.M.F. from 
reversing the direction of current flow outside of the dynamo, a 





FIG. 31. 



FIG. 32. 



''commutator" is used, as shown in elementary form in Figs. 31 
and 32. Here the armature coil is shown with only one turn, the 
armature core is omitted and the magnet poles are drawn further 
apart, for the sake of clearness. The direction of the current flow 
is indicated by arrows. In Fig. 31 the A part of the coil is going 
downward past the vS pole, and in Fig. 32 it is coming upward 
past the N pole. Although the current flow has reversed in the 
coil itself, the flow is not changed in the outside circuit, IV^ be- 
cause the commutator has changed the connections. The strips, 
c and c, are called brushes. 

Q. loi — Has a dynamo armature only one coil? 




FIG. 33. 



A. — No ; it has a great many. Fig. 33 shows a four-coil arma- 
ture, and the number of coils may be made almost anything. In 
practice the number ranges from sixteen to several hundred, ac- 
cording to the size and type of the dynamo. 

O. 102 — Why is the revolving part of a dynamo given the same 
name as the strip of iron in Figs. 20 and 21 ? 



28 



Armatures. 



A. — Because it bears the same general relation to the dynamo 
magnet, known as the field magnet, that the iron bar does to the 
small magnet; namely, it furnishes a good path for the magnetic 
flux passing from pole to pole of the magnet. 




Puiver. y. r. 




FIG. 34. 



FIG. 35. 



Q. 103 — Why is not the armature drawn against the magnet 
poles ? 

A. — Because the pull of one pole is balanced by that of the 
other, the distance between armature and magnet being the same 
on both sides. If the armature is not accurately centered between 
the poles, as in Fig. 34, there is a heavy pull toward the nearest 
magnet pole, which greatly increases the friction of the bearings 
and is liable to spring the armature shaft. 




Power, y. T. 



FIG. 36. 



Q. 104 — Are all dynamo armatures alike? 

A. — No; there are two general classes, the drum armature, of 
which Figs. 31, 32 and 33 are elementary forms, and the ring 
armature, shown in elementary form by Figs. 35 and 36. The ring 
winding is simpler than the drum winding, but not quite so effi- 



Armatures. 



29 



cient because of the greater resistance of the wire for a given 
size of armature. In the ring armature the Hues of force separate 
and travel through opposite halves of the ring-shaped core, as 




FIG. 37. 

indicated in Fig. 37, while the lines go nearly straight through a 
drum core, as shown by Fig. 38. 

Q. 105 — Why are so many wires put on a dynamo armature? 

A. — To obtain the required E.M.F. at a reasonable speed and 
with a reasonable size of machine. The E.M.F. of one wire is 
added to that of any other wire connected in series with it so that 
the direction of flow agrees. In Fig. 31, for example, if the wire, 




Puwer, y. V. 



FIG. 



A, was cutting lines of force and generating an average E.M.F. 
of I volt in the direction of the arrow, and the wire, B, were do- 
ing the same thing, joining the ends by the piece of wire, .r. out- 



30 Annatiire Calculations. 

side of the magnetic field, would cause the two E.2^I.F's to add 
(See Q. 54) and 2 volts would be the E.iM.F. at the free ends 
connected to the commutator. ]\Iost of the wires on an armature 
core are so arranged that their E.AI.F's are added in a similar 
manner. If they were not, the magnet would need to be of enor- 
mous size or the armature would have to run at an enormous 
speed. 

O. 106 — How do the size of the magnet and the speed of the 
armature affect the E.M.F. of the machine? 

A. — The E.]\I.F. furnished by each armature conductor is pro- 
portional to the number of magnetic lines cut per second by that 
conductor and the number of conductors in series multiplied by 
the E.M.F. of one gives the E.jM.F. of the machine. The size of 
the magnet determines the number of magnetic lines in the field, 
and the speed of the armature determines the number of times 
these lines are cut per second by each conductor. If the number of 
lines passing from pole to pole through the armature be repre- 
sented by <I> ; the number of wires all around the outside surface 
of the armature by ic; the number of paths through the armature 
winding by &, and the revolutions per minute by S, the E.IM.F. 
at the brushes will be found by the formula — 

_ 2 X <^ X 2^ X Rev. ^ X cc X Rev. ^ ^ 

E = — r— or r (20) 

6,900.000,000 X '^ 3.000.OCO.000 X V 

0. 107 — What determines the amount of current that a dynamo 
can give? 

A. — The size of the wire used on the armature and the manner 
of connecting the winding. 

Q. 108 — What effect has the size of the wire? 

A. — The larger the wire the more current it will carry without 
overheating. 

O. 109 — How does the size of the wire affect its heating? 

A. — Forcing current through it develops heat, just as forcing 
anything through a pipe would: the electrical resistance of the 
wire being similar to the mechanical resistance of the wall of. the 
pipe. 

Q. no — How much heat is developed in a wire per ampere of 
current ? 

A. — The heat units developed per second are proportional to the 



Temperature of Windings. 



31 



watts passed through the wire. The rise in temperature is pro- 
portional to the watts divided by the affective radiating surface. 

Q. Ill — How can the heat units be calculated? 

A. — As 778 heat units equal i foot-pound, and 1.35625 foot- 
pounds per second equal i watt (see page 13), 1055. 1625 jieat 
■units per second will be developed when i watt is applied to a cir- 
cuit. 

Q. 112 — How can the rise in temperature be computed? • 

A. — It cannot be accurately computed for an armature except 
by comparison with an actual case, because the rapid motion of 
the armature surface increases its radiating capacity. For field 




FIG. 39. 



magnet and other motionless coils the rise in temperature is prac- 
tically equal to 200 times the number of watts divided by the 
radiating surface in square inches. In formula shape, 

200 X f'^ 



= Rise in Fahrenheit decrees. 



O. 113 — How does the manner of connecting the armature 
winding aft'ect its capacity for current? 

A. — If there are two paths through the armature winding, the 
total allowable current will be twice the capacity of the armature 



32 



Multipolar Dynamo. 



wire; if there are four paths, the armature winding will stand 
four times the current that the armature wire can carry without 
overheating. This is simply because the paths through the arma- 
ture winding are in parallel. (See answer II. to Q. 54, page 11.) 

Q. 114 — ^^^hat is meant by the number of paths through the 
armature winding? 

A. — Reference to Fig. 36 will make this clear. Tracing the 
current from the — brush through to the + brush, it will be seen 
that it divides, half going through coils i, 2 and 3, and half 
through coils I., II. and III. There are, consequently, two routes 
or paths through the winding. 




FIG. 40. 



Q. 1 1 5 — Are there ever more than two paths ? 

A. — Frequently. In multipolar dynamos there are usually as 
many paths through the armature as there are poles on the ma- 
chine. 

Q. 116 — What is a multipolar dynamo? 

A. — One having more than two poles. Fig. 39 shows one plan 
of field magnet having four poles. The dotted lines and arrows 
indicate the courses taken by the magnetic lines. The north poles 
may be identified by the arrows passing from the magnet to the 
armature, the south poles being those to which the lines leturn. 



Armature Winding of Multipolar Dynamo. 



33 



Fig. 40 gives a perspective view of a macliine having this type of 
magnet. 

Q. 117 — How is the armature winding of such a machine ar- 
ranged ? 

X 




Power, N. Y. 



FIG. 41. 



A. — A ring winding is arranged exactly the same as in a bipolar 
machine, but two extra brushes are applied to the commutator, as 
shown in Fig. 41, two being positive and two being negative. The 
two positive brushes are connected together and form one positive 
armature terminal; the two negative brushes are similarly con- 
nected and form the negative terminal. A drum winding may be 
arranged exactly as it is for a bipolar field, and the two additional 
brushes used. In practice, however, the coils are wound so that 
when one side is under one magnet pole the other side is under 




the neighboring magnet pole, instead of being diametrically op- 
posite. This arrangement makes the ends of the coils shorter and 
reduces their resistance. Fig. 42 shows a coil consisting of two 
turns of wire, wound for a four-pole field. The dotted lines indi- 



34 



Construction of Armature Core. 



cate the wires across the far end of the core. The connections to 
the commutator are usually the same as in the bipolar machine, 
and brushes are added as in Fig. 41. 

Q. 118 — Is the armature core a solid piece of iron? 






Power, N.T. 



FIG. 43. 



A. — No ; it is always "laminated," which means divided up into 
thin sheets or plates. These plates are insulated from each other 
by a thin coat of varnish or by sheets of tissue paper of the same 
shape as the iron plates, in order to prevent the generation of 
eddy currents in the core. In Fig. 43, A is 3, plain drum core disk ; 
5 is a plain ring core disk and C is a slotted drum core disk, which 
is the kind mostly used in this country. 

Q. 119 — What are the slots for in C? 

A. — To contain the armature wires. Instead of laying them 
along the surface of the core they are put in the slots. 

Q. 120 — What is the advantage of such construction? 

A. — The air gap, or distance between the iron of the armature 
and that of the pole pieces, is considerably reduced, and as air is 




Toiuer, N. F. 



FIG. 44. 



a poor magnetic conductor, shortening this air gap greatly reduces 
the magnetic resistance or reluctance of the path of the magnetic 



Eddy Currents — Commutators. 



35 



flux. Furthermore, the armature wires are better protected 
against mechanical injury and displacement than when on the sur- 
face of the core. 

Q. 121 — What are eddy currents? 




Power, N. Y. 



FIG. 45. 



Power, N. Y. 

FIG. 45a. 



A. — Currents generated in masses of metal which cut lines of 
force, and are allowed to circulate at random. Fig. 44 shows how 
the induced current flows in the wires of an armature ; the current 
is coming toward the observer in the O wires and going the other 

way in the ^Q wires. If the armature core were a solid piece, cur- 




FIGS. 46-47, 



FIG. 48. 



rent would flow from end to end in it just as in the wires. It is 
in order to prevent this that the core is made up of plates or disks 
electrically separated by either enamel or tissue paper. 

Q. 122 — What is the appearance of a complete armature? 

A. — Fig. 45 shows a drum armature and commutator, complete 
with the shaft, and Fig. 45-A shows a ring armature complete. 



36 



Brushes. 



Q. 123 — In Figs. 36, 41 and 45, the parts of the commutator 
are not separated as in Figs. 31 to 35. Why is this ? 

A. — Because a continuous smooth surface is necessary for the 
brushes to rest upon. The copper bars of the commutator are 



FIG. 49. 

electrically separated by mica strips, as shown in the end view 
of the commutator in Fig. 47. Fig. 46 shows the side view, the 
upper half sectional. The heavy black lines represent insulating 
material, such as mica. Fig. 48 shows a complete commutator. 

Q. 124 — Are all brushes flat strips, as in Fig. 32? 

A. — No ; few are. Copper brushes are made up of strips of thin 




Power, N. Y. 



FIG. 50. 



sheet copper, soldered together at one end, as in Fig. 49. The 
free ends are cut to a bevel, as in Fig. 50, to give a large contact 
surface next to the commutator. The position of such a brush 
relative to the commutator is shown by Fig. 51. Brushes are also 
made of carbon blocks, sometimes to the shape shown by Fig. 50, 
which type is set at an angle like a copper brush; sometimes the 




FIG. 51. 




carbon brush is a simple rectangular block and set radially on the 
commutator, as in Fig. 52. 

Q. 125 — Why is carbon used for a brush ? 

A. — Because it reduces ''sparking" or "flashing" as the com- 



Brush Contact — Commutation. 



37 



mutator bars pass the brush. Fig. 53 shows what occurs when a 
bar passes from beneath a brush. In the position shown, the arma- 
ture coil, C, is "short-circuited" by the brush and no current flows 
through it, the currents from the coils on each side of it going 
directly to the brush from the commutator segments, c and d. 
When the bar, c, passes beyond the brush the current flow be- 
tween them is interrupted and the current compelled to go through 
the coil, C, of higher resistance. This causes a spark from c to 
the brush. The higher the resistance of the coil, the worse will 
be the spark. Conversely, the higher the resistance of the bridge 
formed by the end of the brush from c to d, the smaller will 
be the spark. Carbon has a much greater resistance than copper, 
hence its use here greatly reduces the sparking. 




P<ywev,N- Tl 



Q. 126 — Under Q. 117 it was stated that the ends of copper 
brushes were beveled to give a large contact surface. Why is a 
large surface desired? 

A. — To reduce the resistance of the contact and, consequently, 
the heating and loss of energy.* 

Q. 127 — Does not the use of a carbon brush cause heating on 
account of its high resistance? 

A. — It would if the brushes were made of as small area as 
copper brushes. In practice they are much thicker and wider 
than copper brushes would be under like conditions. 



* Commonly called the C^ R loss, because C^ X R 
work or expended energy. 



watts, and watts X seconds =r 



38 



Adjustment of Brushes. 



Q. 128 — Is there any guide for the area of the brush contact? 

A.— Yes ; a series of experiments has demonstrated that the 
area of a copper brush contact should not be less than 4 square 
inches per 1000 amperes, and that of a carbon brush should not 
be less than 2 f square inches per 100 amperes. Expressing it 
differently, the current should not exceed 250 amperes per square 
inch for copper, and 2)72 amperes for carbon brush contacts. As 
the thickness of a brush should not be greater than two to three 
times the thickness of one commutator segment, the desired con- 
tact area is obtained by using several brushes side by side, as 
shown in Fig. 54. 



BRUSHES 




FIG. 54. 



FIG. 54a. 



Q. 129 — What determines the points of the commutator upon 
which the brushes should touch? 

A. — The field magnet. Each brush must touch at such points 
that the coil, which is short-circuited by it, as in Fig. 53, is cutting 
the smallest possible number of lines of force. In a bipolar dy- 
namo the wires represented by plain circles in Fig, 44 are in this 
position, and the bars to which they are connected will be those 
upon which the brushes must rest at the instant referred to by the 
drawing. The points of contact are sometimes called "neutral" 
points, meaning that the coils which the brushes short-circuit at 
those points are neutral, or not generating any E.M.F. The 
brushes are always mounted on a rocker-arm or a ring, which 
enables one to shift them around the commutator and find the non- 
sparking position. (See Fig. 54-A.) 



Shunt Winding. 



39 



Q. 130 — Where does the field magnet get the current to excite 
its coils? 

A. — From its own armature. The whole current sent out by a 
"series-wound" dynamo passes through its field magnet coils, as 
shown by Fig. 55. Hence the name "series wound" — the magnet 
coils are in series with the work circuit and armature. In a 
"shunt-wound" dynamo, only a small part of the total current 
passes through the magnet coils, as they are of high resistance 





FIELD WINDING 



MAIN CIRCUIT 

FIG. 55. 



Power, N. Y. 




MAIN CIRCUIT 



Power, N.T. 

FIG. 56. 



and connected so as to form a "shunt" to the outside circuit, as in 
Fig. 56. The lower diagrams show more clearly the electrical re- 
lations of the dynamo windings to the outside circuits. 

Q. 131 — What is meant by a shunt? 

A. — A by-path in parallel with the principal circuit is a "shunt." 
In Fig. 56 the outside circuit is the principal one, and the field 
winding is therefore a "shunt" as compared with it. 



40 



Rheostats. 



Q. 132 — When a dynamo is at rest and no current is flowing, 
how is the field magnet magnetized ? 

A. — All iron masses retain a small amount of magnetism after 
having once been magnetized. This "residual" magnetism in- 




MAIN CIRCUIT 



Power, N.T. 



FIG. 57. 



duces a very feeble E.M.F. in the armature, and a Httle current 
passes through the field-magnet coils, increasing the magnetism. 
This, in turn, increases the flow of current, and the "excitation" 
of the field continues to increase to the normal point. When a 




FIG. 58. 



dynamo is first tested after building, the exciting current is ob- 
tained from some other source. 

Q. 133 — How is the E.M.F. of a dynamo regulated? 

A. — By varying the strength of the field magnet. In a shunt- 
wound dynamo this is accomplished by putting resistance coils in 



Series Field Winding. 



41 



series with the field winding, and cutting in or out the resistance 
coils by means of a metal contact finger {s, in Fig. 57) traveling 
over metal contact buttons to which the resistance coils are con- 
nected. The complete apparatus is called a rheostat ; Fig. 58 



<n 


^0 — 
— 

-— — 


^ I" 








1 




<io'^ 






< "' 


\ 


rP 


^ 


L 




-,r-v-,r^r-v-^ 



FIELD WINDING 



MAIN CIRCUIT 
FIG. 59. 



shows a modern rheostat. Series-wound dynamos are regulated 
sometimes by a rheostat connected in shunt to the field-magnet 
coils, as in Fig. 59. The current passing from a to h divides be- 
tween the magnet coils and the rheostat coils ; the higher the re- 
sistance of the rheostat the less current passes through it, and the 
more through the magnet coils, hence the stronger the field mag- 
net. Another method consists of cutting in or out of circuit sec- 
tions of the field magnet winding, as in Fig. 60. The object of all 
these methods is to vary the strength of the field magnet, and, 
thus, the E.M.F. of the armature. (See page 30.) 

Q. 134 — What is the reason for using different field windings? 

A. — To suit the machine to different requirements. A series- 




Power, N. F. 



FIG. 60. 



wound dynamo is used when the current in the main circuit is to 
remain unchanged, because the E.M.F. at the brushes is always 
very high — usually several thousand volts — and if the field coils 
were connected in shunt their resistance would have to hii very 



42 



Compound Field Winding. 



high, necessitating- the use of very fine wire, so that the cost of 
wire would be much greater than with series coils of heavy wire. 
As the current does not fluctuate very much, the whole of it can 
be passed through the field coils. Such an arrangement is known 




MAIN CIRCUIT 




SERIES 
COILS 



MAIN CIRCUIT 




Power, N.r. 



m^LsmMMMmu 



SERIES 
COILS 



SHUNT COILS 



MAIN CIRCUIT 



FIG. 62. 



l\)U)er,Is:r. 



FIG. 61. 



as a constant-current system. It is used in arc lighting, where 
all the lamps are connected in series, as in Fig. 9. 

O. 135 — How is a shunt-wound dynamo used? 

A. — For supplying a "constant-potential" system, such as in- 
candescent lamp circuits (see Fig. 11), and power circuits. Here 
the E.M.F. or potential* remains practically unchanged and the 



A contraction of the term "difference of potential." 



Compound Field Winding. 



43 



current varies according to the load. Hence, a series-wound dy- 
namo could not be used satisfactorily because its field strength 
would fluctuate with the load, and when the latter fell to a small 
value, the dynamo field would be too weak to keep up the E.M.F. 

Q. 136 — Are there any other forms of field magnet winding? 

A. — Yes ; compound windings are extensively used for constant 
potential dynamos. Fig. 61 shows the usual arrangement. The 
field magnet windings are divided into two parts, fine wire coils in 
shunt to the circuit, marked ''shunt coils" in the lower diagram, 
and heavy wire coils in series, marked "series coils." The shunt 
coils supply most of the field strength, and their effect is regulated 




FIG. 63a. 



by a rheostat, exactly as in the plain shunt-wound dynamo. In 
some cases the shunt coils are connected in shunt to the brushes, 
as shown in Fig. 62; such an arrangement is known as a "short 
shunt" connection, and that of Fig. 61 is called a "long shunt," 
because it shunts both the armature circuit and the series coils. 

Q. 137 — What effect do the series coils give? 

A. — They serve to increase the total field strength as the load 
increases. Thus, when there is no load the shunt coils excite the 
field alone. As the load comes on, the main current flowing 
through the series coils adds to the field strength; and any in- 
crease in load will increase the field strength some, but not in di- 
rect proportion, of course. 



44 



Principle of Motor Operation. 



Q- 138 — What is the object of increasing the field strength with 
the load ? 

A. — To increase the E.M.F. generated in the armature, and 
thus compensate for the loss in the armature circuit. (See Q. 200.) 

Q- 139 — What is the difference between a dynamo and a motor ? 

A. — Practically, there is none. The construction is substan- 
tially the same, and a dynamo may be used as a motor or vice versa. 

Q. 140 — How is it that the same machine can be used either 
to generate current or to be driven by the current ? 

A. — Because any machine that will convert motion into another 
form of energy, will, if supplied with that form of energy, con- 




Power. N.Y 



FIG. 64. 



vert it back into motion. Consider the case of an air compressor, 
power pump, steam engine or any other machine for changing 
energy from some other form to motion or vice versa. A hy- 
draulic pump, if supplied with water under pressure, will be 
driven by it as a motor ; so also will an air compressor. 

Q. 141 — How does the current cause a motor armature to turn ? 

A. — Part of the current is used to magnetize the field magnet ; 
the remainder goes through the armature and, in effect, mag 
netizes it, and the two magnetic fields that are set up attract each 
other and cause the armature to turn. Reference to Figs. 63 to 
6y and the answer to Q. 76 will explain. Fig. 63 shows a single 
coil wound on an iron core which is free to revolve around its 



Function of the Commutator. 



45 



center. Passing a current through the coil creates magnetic 
"poles" at n and s. Now mount this core between two magnet 
poles, as in Fig. 63-A, and it will turn until its n pole is opposite 
the vS pole of the magnet. 




FIG. 65. 



Q. 142 — Will it not come to a rest in that position ? 
A. — It would, but for the commutator. Fig. 64 shows the one- 
coil armature with a commutator. The brushes are so set that the 




FIG. 66. 



current is reversed in the coil just as n and ^9 are opposite, so that 
n becomes i- and repels the S pole of the magnet. 

Q. 143 — How is it when several coils are used on the armature 
core ? 



46 



Counter E.M.F. 



A. — The combined effects of all the coils result in setting up a 
magnetic field corresponding with that of a single coil. Fig. 65 
shows four coils, a, b, c and d; a and h set up a field at n, and 
an opposite one at s; c and d set up similar fields at n\ and s\ and 
these two sets combine in two fields, each spread around nearly 
one-half of the core, with centers at nn and ss — just where the 
single coil in Fig. 63 set up its fields. Fig. 66 shows the armature 
turned to where the coils, a and h, are "cut out" by the brushes, 
and Fig. 6^ shows it turned a little further, with the resultant 
field centers, nn and ss, shifted backward. 




FIG. 67. 



Q. 144 — Then without a commutator a motor could not run ? 

A. — No ; just as an engine could not run without a valve to re- 
verse the application of steam to the piston when a dead center is 
reached. The commutator performs this function for each coil of 
the armature as soon as its magnetic axis coincides with that of 
the field magnet poles. 

Q. 145 — What prevents a motor from running away when the 
load is taken off? 

A. — The armature wires generate an E.M.F. when they cut 
the magnetic lines of the field, just as in a dynamo. This E.M.F. 
is opposite in direction to the line E.M.F. which drives current 
through the armature so that it keeps the current down, and this 
keeps the speed down. This refers to a shunt- wound motor 
operated on a constant-potential circuit. 



speed Variation of Shunt Motor. 47 

Q. 146 — Does not the speed of a constant-potential motor ever 
change ? 

A. — Yes, a Httle. If the armature windings had no resistance 
the speed would be constant. 

Q. 147 — How does the resistance aflfect the speed? 

A. — Part of the line E.M.F. is used up in forcing current 
through the armature resistance; this part, frequently called the 
resistance-volts, is equal to C X ^ ^ '^. The remainder {E — v) 
must be exactly balanced by the back E.M.F. {e) of the motor. 
Now, the motor E.M.F., e, varies with the speed; hence when v 
increases, e must decrease, because e -]- v must equal the line 
E.M.F. For example, if C = 10 amperes and i^ = ^ ohm, v will 
be -J X 10 = 5 volts. If E is 100 volts, the motor E.M.F., e, must 
be 100 — 5 r= 95 volts. Now, if the load doubles, it is evident 
that 10 amperes will not pull it; the armature will slack up until 
the back E.M.F., e, drops to 90 volts, so that the resistance-volts, 
V, will be 10 volts and C will be 20 amperes, or twice its former 
value. The speed will now be ^i or -i| of its first value. 

Q. 148 — Is the motor E.M.F., e, always equal to the line E.M.F. 
minus the volts used up by resistance? 

A. — Invariably. 

Q. 149 — What determines the speed at which a motor will 
run ? 

A. — The strength of the field, the number of armature vv^ires 
and the resistance of the armature circuit from brush to brush. 

Q. 150 — How can the speed be calculated ? 

A. — First find v by multiplying the resistance by the current 
that the armature will stand. Subtracting this from the line 
voltage leaves the value of the back E.M.F., e. This is the product 
of speed, wires and magnetic field, as in a dynamo (see Q. 106), 
so that multiplying the wires by the field and dividing 6,000,000,- 
000 times e by the result gives the speed. A formula is easier to 
grasp. Let E represent the line voltage; e the back E.M.F. of 
the armature ; R its resistance ; C its current, h the number of 
paths through the winding; w the number of wires around the 
outside ; $ the number of magnetic lines of force passing from 
the field through the armature, and Rev. the revolutions per 
minute. Then the back E.M.F. will be 



4^ Motor Torque. 

<^X w X Rev. 



3,000,000,000 X b 

Now, e is also equal to E — C X -^, so that 

^ X w y. Rev. 



(20a) 



E—CXR = 



3,000,000,000 X b 

Therefore, 

^ 3,ooo,ooo,oooX^ X (^ — C X Tv") , ^ 

Rev. = ^ ~ (21 ) 

<^ Xw ^ ^ 

Q. 151 — What determines the amount of load a motor can pull? 

A. — The number of armature wires, current and magnetic flux 
determine the pull in pounds at the surface of the armature, and 
the speed fixes the foot-pounds per second. The pull at the sur- 
face multiplied by the radius of the armature is called torque, and 
is expressed in pound-feet. 

Q. 152 — What is the difference between foot-pounds and 
pound-feet ? 

A. — Foot-pounds are the product of weight or pull X distance 
of travel. Pound-feet are the product of weight or pull X lever- 
age in feet. For example, if the total pull at the surface of a 
motor armature of 6 inches radius were 1000 pounds, the torque 
would be 500 pound-feet. If the motor speed were 600 revolu- 
tions a minute, or 10 a second, the "distance of travel" of any 

' , , , , 6.28-^2 X 6 ins. X 10 

pomt on the surface would be 1. = 3i.4it) 

12 

feet a second, and multiplying this by the pull of 1000 pounds 
gives 31.416 foot-pounds per second. From which it will be 
seen that pound-feet and foot-pounds are proportional to each 
other, but the ratio varies with the speed. The relation may be 
analyzed as follows : 

The circumference of an armature, in feet, X revolutions per 
second ^ distance of travel per second. This multiplied by the 
pounds of pull (P) gives foot-pounds per second. The circum- 
ference = 3.1416 X diam., or 6.2832 X radius. So, if rev. is the 
number of revolutions per second and r the radius in feet (or 
fractions of a foot), 

6.2832 X ^ X '^^^- X i^ == Ft. Lbs. per sec. 
Now the torque is equal to P X ^. as just stated, hence if r repre- 



Motor Torque and Horse-Power. 49 

sents pound-feet it can be substituted for P and r, and we get 

6.2^2>^ X 'rev. X T ^ Ft. Lbs per sec. 

It is customary to represent (6.2832 X rev.) by the symbol co ', 

using this, the formula becomes 

ft)XT = Ft. Lbs. per sec. 

Q. 153 — What is the formula for the horse-power of the motor ? 

A. — This may be expressed in several ways. Foot-pounds per 

second are changed to horse-power by dividing by 550, hence 

6.28^,2 X rev. X r rev.X t wxt 

^ , or , or = H. P. 

550 «7.535 550 

Q. 154 — How can the torque be ascertained? 

A. — If the magnetic flux ($) is known, the torque can be 
computed from the formula 

$ X ^ X C ^ 

426,096,000 ^ '^ "^' 

Stated as a rule : 

Multiply together the magnetic flux through the armature, the 
number of wires around the outside (or in slots) and the total 
armature current ; divide the product by 426,096,000. 

Q. 155 — Suppose the magnetic flux is not known? 

A. — The resistance of the armature is easily ascertained, either 
from the builders or by measurement. Having this, the horse- 
power may be computed without reference to the torque. Multi- 
plying the resistance by the current gives the wasted volts; sub- 
tracting these from the line E.M.F. leaves the back E.M.F. of 
the motor. Multiply this by the armature current and the product 
will be the output in watts ; divide this by 746 and you have the 
horse-power. The formulas are 
E — CR = e, and 

eXC 

-74^-^^ (^4) 

Q. 156 — Can the speed of a motor be regulated? 

A. — Yes; in two ways. The E.M.F. applied to the brushes, 
which is ordinarily considered as the line E.M.F., can be cut down 
by putting resistance in series with the armature, as in Fig. 68. 
This pulls the speed down, just as the armature resistance does 
(see answer to Q. 147). The speed can be regulated also by vary- 
ing the strength of the field magnet, which results in varying the 
back E.M.F., e. 



50 



Regulation of Speed. 



Q. 157 — How does the last method affect the speed? 

A. — Strengthening the field reduces the speed, and weakenmg 
it increases the speed. Reference to formula (21) will show 
mathematically the truth of this. <i> represents the magnetic flux, 
and if this is increased without any other change, the value of 
Rev. will be diminished. Simple reasoning, without any mathe- 
matics, also shows it to be true. We know that the back E.M.F. 
can not exceed the difference between the lost volts (C X ^) and 



> 




Kmmsmmimu. 

FIG. 68. 



the line E.M.F. ; hence, with a constant line potential and a con- 
stant load the back E.M.F. must remain constant. We further 
know that the back E.M.F. (the number of wires being constant) 
varies with the magnetism and the speed, so that in order to pre- 
serve a constant back E.M.F. the speed must be reduced if the 
magnetism is increased, and vice versa. 

Q. 158 — Then, as the speed increases when the magnetism de- 
creases and vice versa, would not a motor run faster with no field 
magnetism and be brought almost to rest by making the field 
strong enough? 

A. — No. The speed can not increase beyond the point at which 
the pull between the armature wires and the field magnetism is 



Motor Starters. 51 

strong enough to enable the armature to pull its load at the in- 
creased speed. And in the other direction, the magnetism can not 
be increased beyond the ''carrying ability" of the iron cores 
(see Q. 77)- ^ 

Q. 159 — Which way is the speed of a motor usually regulated? 

A. — By means of a rheostat in series with the armature. Field 
regulation is employed in some special cases, but the other is the 
common method. 

Q. 160 — How is a motor connected to the circuit? 

A. — As shown diagrammatically by Fig. 69. The "starting box" 
is a rheostat, the resistance coils of which are in series with the 
armature at starting. They are cut out by means of the hand 
lever as the motor gains speed, until all are cut out of the arma- 
ture circuit. 

Q. 161 — What are the devices indicated by M and Sf 

A. — Magnets. The one at 5' is usually a solenoid ; it operates 
to open the circuit if the current exceeds a certain strength. The 
one at M holds the hand lever in the proper position for running 
until the current ceases ; then it releases the lever, which is pulled 
back to the starting point by a spring. 

Q. 162 — How are the connections arranged inside the box? 

A. — As shown by Fig. 69, which also shows the mechanical ar- 
rangement of the levers and magnets. When the current becomes 
too great, the solenoid, S, trips the latch, /, and this releases the 
lever, A, which is pulled by the coil spring away from its con- 
tact button (indicated by the dotted circle) and opens the circuit. 

Q. 163 — How does the magnet, M, operate? 

A. — It is connected in series with the motor field winding, and, 
therefore, any interruption of the current flow as a whole or of 
that portion in the field circuit, will "kill" the magnet and allow 
the lever, L, to be thrown to the starting position, where the 
whole motor circuit is opened. 

Q. 164 — Why is the magnet connected in the field circuit? 

A. — Because there it protects the motor not only in the event 
of a general cessation of the line current, but in case of accidental 
interruption of the current through the field circuit. If the mag- 
net were in the armature circuit a break in the field circuit could 
not affect it. 



52 



Motor Starters. 



Q. 165 — Why should the motor require to be cut out in the 
event of a failure of the line current? 

A. — In order that it may not be damaged when current is re- 
stored to the line with the armature at a standstill. 

Q. 166 — What would be the result if the field circuit were 




FIG. 69. 




Puwer, ^f. Y. 



FIG. 70. 



opened with the motor running, if the release magnet and spring 
were not there? 

A.— The magnetism would cease and consequently the arma- 
ture would come to rest. The current through it would rise at 
■once to an enormous overload and cause damage, provided the 
•overload solenoid, S, were also absent. With the solenoid there 
it would open the motor circuit. 



Motor Speed Regulation. 



53. 



Q. 167 — Are all starting boxes made like Fig. 69? 

A. — No. Many of them have only the *'no-current" release 
magnet, as shown in Fig. 70. Fig. 71 shows an exterior view of 
such a box. It is, of course, preferable to have both the no-cur-- 
rent release and the overload release. 



Vo°°°On© 




Fbwer.N.r. 




FIG. 71. 



FIG. 72. 



Q. 168 — How are motors arranged when field regulation is 
used? 

A. — In a variety of ways. Fig. y2 shows, in diagram, a 




method used occasionally. A rheostat is placed in the field circuit 
and adjusted to vary the field strength just as in the case of 
a dynamo. Another method of field regulation consists of 
dividing the field coils into several distinct windings and con- 
necting or disconnecting the various windings, as indicated by 



54 



Commutated Field Winding. 



Fi§^- 73' An elaboration of this plan consists of grouping the 
field windings variously by means of a special type of switch 
known as a controller. At the start all the windings are in series 
with each other, as in Fig. 74. The next movement of the switch 



WW mAT^msj mm 



Power, If. r. 



FIG. 74. 



puts the windings partly in series and partly in parallel, as in Fig. 
75, and so on, until the final movement puts them all in parallel 
with each other and in series with the armature as in Fig. 76. 
Each successive step reduces the resistance of the group and al- 
lows the armature to take a higher speed. 

Q. 169 — Is there not a change in the field magnetism when the 
field windings are grouped differently? 

A. — Yes ; the field is weakened with each change, in the pro- 
gression from all-in-series to all-in-parallel, if the requisite torque 
remains the same or is diminished. For example, suppose for 



3- 




FIG. 75. 



simplicity that a motor has a load requiring constant torque or 
"pull," at all speeds. Also assume that with full current the field 
magnet core is not quite saturated under the all-in-series condition 
of Fig. 74. Now put all of the coils in parallel, as in Fig. 76. 



Comnmtated Field Winding. 



55 



To give the same field strength as before, the current per coil must 
be the same, which would make the total current four times as 
great. But with four times the current in the armature the 
torque will be four times as great, as 

^X wxc _ 

426,096,000 
In order to preserve the same torque (the torque was assumed 
constant), the field must weaken as much as the armature current 
increases ; i. e., the current will increase to twice its original value 
and this will give the field winding J its original ampere turns. 
Hence, <J)will be about -J its first value while C is doubled. The 
steps between the two extremes of Figs. 74 and y6 are merely 
progressions in the direction of the final result. 




FIG. 76. 

Q. 170 — How is the speed affected? 

A. — It is increased by reason of the two effects : the weakening 
of the field magnetism and the reduction of the resistance in 
series with the armature. When the field coils are grouped as in 
Fig. y6, their joint resistance is one-sixteenth of the resistance 
when grouped as in Fig. 74. Hence, the armature must run 
faster in order to generate sufficient counter E.M.F (e) to make 
up the difference between the volts lost in the motor windings and 
the line E.M.F. And as the field is weakened, the speed must be 
still greater in order that the back E.M.F. may equal E — v. 

Q. 171 — In the case illustrated, what would be the increase in 
speed ? 

A. — Assuming that at maximum current load (Fig. y6) the 



56 



Differential Field Winding. 



volts lost in the armature are 2 per cent and those lost in the field 
are 5 per cent of the line E.M.F. ; then, if the torque required by 
the load be constant, the speed will be a trifle over three times 
as great for Fig. 76 as for Fig. 74. 

Q. 172 — Are motors ever given compound field windings like 
dynamos ? 

A. — ^Yes. When a motor has both series and shunt windings^ 
however, they are usually connected so that the series winding 
tends to demagnetize the field, as in Fig. yy. This is done in 
order to maintain a very even speed. 

Q. 173 — How does such a winding maintain a constant speed? 

A. — As the load increases the current in the series coil increases 
and weakens the field just enough to make up for the loss of speed 




which would be caused by the increased volts lost in the armature 
by reason of the heavier current flowing. For ordinary pur- 
poses a plain shunt-wound motor runs at sufficiently even speed, 
the variation usually being well within 5 per cent. 

Q. 174 — What is the winding in Fig. yy called? 

A. — Differential field winding. 

Q. 175 — How is that of Figs. 74 to yS distinguished? 

A. — It is known as a "commutated" field. 

Q. 176 — In what class of work is commutated field regulation 
used ? 

A. — It is employed to control the speed of motors used in work 
which requires them to run continuously at any one of several 
speeds, or to start under heavy load, as in elevator and street 
railway service. 



Reversal of Rotation. 



S7 



Q. 177 — Does not an elevator motor require to be reversed? 

A. — Yes ; when the motor drives the elevator winding-drum or 
sheaves direct or through rigid gears, as all efficient electric ele- 
vator rigs are arranged. 

Q. 178 — How can a motor be reversed? 

A. — By simply changing the connections between the brushes 
and the terminals and leaving the field connections unchanged. 
For example, if the motor in Fig. 78 runs in the direction indicated 
by the arrow when the connections are as shown, it can be made to 




FIG. 78. 

run in the other direction by connecting the wire, a, to the brush, 
B, and the wire, h, to the brush, A. 

Q. 179 — How does the controller of a commutated field motor 
change the field connections? 

A. — Each terminal of each coil, each brush connection and each 
outer terminal of the entire motor is led to a spring contact finger. 
These fingers are arranged in a straight line parallel with the 
axis of a drum carrying contact strips. Rotating the drum brings 
these strips in contact with the fingers, and at successive positions 



58 



Motor Controller. 



of the drum the connections between the fingers are changed to 
give the successive combination of circuits. Fig. 79 shows such 
a controller. The relation of each contact finger to the drum 
and its strip is shown by the sketch at the bottom of the cut. 

Q. 180 — Are there any other methods of regulating the speed 
of a motor? 

A. — There is one other method, which is In extensive use for 




Fig. 79 




speed regulation of electric railway motors. This method con- 
sists of grouping two motors first In series and then In parallel, 
and varying a small amount of dead resistance in series with 
them. A controller similar to Fig. 79 is used. Fig. 80 shows 
diagrammatlcally the successive arrangements of the circuits, 
from the starting point to full speed. The combinations are in- 



Series-Parallel Control. 



59 



A rm/mmi \Mvw~^MwrY(3/^~^^ 



-TERMINAL 



TERMINAL- 



B ~\wMMwvrpww — \m^N^^Y^^y^ 



-TERMINAL 



TERMINAL- 



c -^NwmN^Nr'^^mrTy\N^^ 



TERMINAL- 



TERMINAL- 



:D ~^/WV\WAWr~AA/VW ^^ 



E "WWWWWV — \i 




TERMINAL 



TERMINAL ^ 




•TERMINAL 



TERMINAL- 



G ~~\tmAmi\N' 




TERMINAL 



TERMINAL- 




TERMINAL 



FIG. 80. 



6o Series-Parallel Control. 

dicated by the letters, A, B, C, etc. The zig-zag lines represent 
resistance coils and the spirals represent field coils. 

Q. i8i — Why are resistance coils connected in parallel with 
the field coils in the E and / combinations? 

A. — To shunt part of the current out of the field magnet coils 
and weaken the field, increasing the speed of the motors. The R 
combination gives the greatest speed obtainable with the motors 
in series and the / combination the maximum speed in parallel, 

Q. 182 — What is the difference between the two speeds? 

A. — That given by / is about twice that given by E, because at 
/ the resistance-volts and back E.M.F. of each motor must equal 
the line E.M.F., while at E the resistance-volts and counter 
E.M.F. of each motor equals only one-half of the line E.M.F. 
At J , however, the resistance volts per motor are slightly greater 
than at E, hence the speed is not quite doubled. 

Q. 183 — Why are the resistance- volts per motor greater at / 
than at £.^ 

A. — Because the motors are doing more work (driving the car 
at the higher speed) and the output (C X ^) is greater, calling 
for more current. And as C X -^ = resistance-volts (z/), v will! 
be greater than before. 

Q. 184 — The diagram shows series field coils ; if resistance is tO' 
be used, why are shunt field coils not used? 

A. — Because the motors must start under heavy loads, re- 
quiring great torque, and series field coils best fulfil this condition. 
Moreover, it is advisable to have the torque increase as much as 
possible with the load, and the maximum increase is given by 
series coils, because when the armature current increases the field 
magnetism also increases, and torque is directly proportional to 
Magnetic Flux X Current, as shown by formula (23). In an 
ordinary shunt-wound motor the field magnetism remains con- 
stant, only the armature current varying with the load. 

Q: 185 — Could not a railway motor be wound with series coils, 
for starting and shunt coils for running at full speed? 

A. — Yes ; motors have been so built, but the slight advantage 
gained does not seem to justify the extra expense and complica-^ 
tion involved in such design. 

Q. 186 — How is the power of a railway motor transmitted ta 
the car axle? 



Railway Motor Connection. 6i 

A. — Through spur gears enclosed in a casing which serves the 
<louble purpose of containing a lubricant for the gears and muf- 
fling their noise. 

Q. 187 — How are railway motors connected in circuit when 
there are many cars ? 

A. — The individual motors of each pair are connected in series 
and parallel with each other, as previously described, but eacli 
complete pair constitutes a unit, and the units (car equipments) 
are in parallel between the circuit wires, which are nominally at 
constant potential. 



CHAPTER IV- 
CIRCUITS AND WIRING* 

Q. 1 88 — How is connection made between the car and the cir- 
cuit ? 

A. — One terminal of the motor equipment is connected to a trol- 
ley wheel carried at the end of a pole on top of the car. This 
wheel rolls along a "trolley" wire suspended over the center of 
the track, and connected with one terminal of the dynamo. The 
other terminal of the dynamo is connected to the track and the 
car wheels form the connection between this and the other ter- 



TROLLRY WIRE 



n 



^ (o}_ 



(5) (5) 



FIG. 81. 

minal of the motor equipment. See Fig. 8i, where the generator 
is represented by G. Fig. 82 shows roughly the car circuits; a 
controller stands on each platform and wires connect both con- 
trollers with the motors, M^ M, in accordance with the require- 
ments of Fig. 80. 

Q. 189 — What are the circles marked Lf 

A. — Incandescent lamps for illuminating the car. 

Q. 190 — Why are they connected in series? 

A. — Because the circuit E.M.F. is too high to connect them in 
parallel. The E.M.F. commonly employed is 500 to 600 volts, and 
no incandescent lamps are made to stand that potential. 

Q. 191 — Are several dynamos worked together on a single 
circuit, like motors? 

A. — Frequently ; especially on constant-potential circuits. Fig. 
S>^ shows diagrammatically two shunt-wound dynamos connected 



Dynamos in Parallel. 



63 



to one circuit ; A and B are the commutators (the armatures are 
omitted for simplicity) and the horizontal lines marked + bus- 



TROLLEY WIRE 




TO WHEELS 



FIG. 82. 



bar and — bus-bar are heavy copper rods, to which the dynamos 
and circuits are connected in parallel. Any number of constant- 
potential dynamos may thus be worked in parallel, provided they 
all give the same E.M.F. 



■4- BUS BAR 



BUS BAR 



+ 



^^n:^, 



RHEOSTAT 
FIELD COILS 



-t- 



B 



\tS^rhe( 




Power, H.y, 



FIG. 83. 



Q. 192 — Are constant-potential dynamos ever worked in series? 
^^- — Not strictly in series in the ordinary sense. They are 



64 



Three-Wire System. 



connected in series, with a third wire, called the "neutral wire,'* 
leading out from the connection between the dynamos, as in Fig. 
84. Such an arrangement is called the "three-wire system," and 
is virtually two plain parallel systems combined by consolidating 

M' 





® ® © $) 



FIG. 84. 

one leg of one circuit with one leg of the other circuit. The 
dynamos are in series so far as the outside wires are concerned, 
and the wires and lamps constitute a series-parallel system made 
up of two sub-groups. In the diagram the dynamos are repre- 
sented by the circles, G and C Fig. 85 shows a hydraulic ana- 
logue in which pumps and tanks, P and R, correspond with the 
dynamos, pipes correspond with the wires and water motors with 
the lamps. It is evident that water will flow through both groups 
of motors in series and through both pumps and tanks. Current 
passes through the dynamos and lamps in the same way. 
Q. 193 — Why is the third wire in the middle necessary? 




Power, N. Y. 



FIG. 85. 



A. — To give control of each lamp separately. The lamps could 
be connected two in series between the outside wires and the mid- 
dle wire left out ; in this case, however, the lamps must be lighted 
and extinguished in pairs. Such a system has been used, how- 



^hree-Wire System. 



6s 



ever, to a limited extent, one 250-volt dynamo being employed 
instead of two of 125 volts each. 

Q. 194 — When part of the lamps between the middle wire and 
one outside leg are cut out, what happens? 

A. — The difference between the two pairs of the whole load is 
taken care of by the neutral wire. In Fig. 86, the generator, G\^ 
supplies five lamps, while G supplies eight. Assuming -J ampere 
per lamp, the wire, M, carries out 2-J amperes, the neutral, N, 
carries out ij amperes, and these two unite and return by way of 
Ml, which carries 4 amperes. If the neutral, N, were cut and 
only 2-| amperes were sent through the wires, M and M^, the 
5-lamp group would burn all right, but the 8-lamp group would 
simply turn dull red. And without the neutral wire it is evidently 
impossible to get 4 amperes through the big group without pass- 




06600600 



N 



6 6 



M 



Power, N, T. 



FIG. 86. 



ing it also through the small group, which would destroy the 
lamps of that group. 

Q. 195 — What regulates the division of the current? 

A. — The resistance of the load. The E.M.F. between M and 
A^ is maintained constant at the lamps ; that between N and M-^ 
is similarly controlled. Hence, the amount of current flowing be- 
tween either outside leg and the neutral is determined by the re- 
sistance between that leg and the neutral regardless of what exists 
between the other leg and the neutral. The difference between 
the current in one leg and that in the other is carried by the neu- 
tral. 

Q. 196 — Are series-wound dynamos run in series or in parallel? 

A. — When they are combined on one circuit, which is seldom 
done, they are connected in series, as shown by Fig. %J. The 
squares, marked S, are brass plates with taper sockets. Con- 



66 



Scries Dynamos — Compound Dynamos. 



nection between any two sockets is made by a flexible conducting" 
cord, each end of which is attached to a brass phig, fitting snugly 
in the sockets. The dotted lines indicate the connections made by 
conducting cords in the diagram, and the arrows indicate the flow 
of the current. 

Q. 197 — How are compound- wound dynamos operated to- 
gether ? 

A. — Compound-wound dynamos are operated in parallel or on 
three-wire systems, like plain shunt-wound machines. When 
operated in parallel an extra connection is necessary, as shown at 



-K 






c 


1 




i 


V 00000 


1 


o]S 


FIELD COILS 


X 




) 


so J 



Slo] 
t 



Power, S.T. 



TO OUTSIDE 
CIRCUIT 



FIG. 87. 



E, Fig. 88, allowing current to flow from the brush end of one 
series coil to the brush end of the other. These connections lead 
to a bus-bar like the two main bus-bars, so as to avoid manipula- 
tion at the dynamos. This bus-bar is called the ''equalizer bus- 
bar." 

Q. 198 — What is the object of this third connection? 

A. — To equalize the current in the series coils of the dynamos. 
The effect of this connection, electrically, is shown by Fig. 89; it 
puts the series coils in parallel with each other, so that even if 
one armature furnishes a little more current than another, the 



Compound Dynamos in Parallel. 



67 



current divides up evenly between the series coils and strengthens 
the weaker armature. If this connection were omitted and the 



POSITIVE BUS BAR 



4 r 



EQUALIZER 



< — e 



BAR 



ERIES 



NEGATIVE 




4 4 



SERIES 
/ SHUNT I / SHUNT 




Power, N.T, 

FIG. 88 



shunt fields were unevenly adjusted so that one armature gave 
more current than the other, this current passing through the 



POSITIVE 




NEGATIVE 
FIG. 89. 



Power, N.r. 



series coils of the stronger dynamo would still further increase 
its strength and throw it still further out of balance with its mate, 



68 Voltage "Drop." 

or mates. Under these conditions lack of balance is dangerous. 
(This is explained in the Appendix.) 

Q. 199 — Why are compound windings used, when they cause 
extra complication? 

A. — Because they increase the E.M.F. of the dynamo as the 
load increases, and thus make up the "drop" in volts in the arma- 
ture winding and in the wires between the dynamo and the actual 
load, doing away with the necessity for regulating by hand every 
time the load changes. The series coils regulate automatically; 
when the load increases these coils increase the field magnet 
strength and raise the E.M.F., and when the load decreases their 
effect decreases, lowering the E.M.F. again. 

Q. 200 — What is the "drop" in the armature? 

A. — The E.M.F. used up in forcing the current through the 
wire — the resistance-volts explained under Q. 147 with regard 
to motors. Suppose a dynamo to be driven at full speed with the 
work circuit disconnected and a voltmeter applied to the brushes 
shows no volts. Now, if the circuit is closed and a load of, say, 
100 amperes results, the E.M.F. at the brushes will not be no 
volts. If the resistance of the armature circuit be ^h of an ohm, 
then, by Ohm's law, t^t7 X 100 {R X C) =3 volts will be used 
to drive the 100 amperes through the armature circuit, and these 
3 volts subtracted from the original no leave only 107 volts at the 
brushes available for outside work. Now, if there is a long stretch 
of wire between the dynamo and the lamps, and the resistance of 
this wire is h of an ohm, then ^V X 100 = 4 volts will be used 
up, forcing the 100 amperes through the conecting wires, leaving 
only 103 volts useful at the lamps. This loss is called "drop," 
because there is a drop in the electrical pressure from the E.M.F. 
generated to that available, corresponding to the drop in steam 
pressure between a boiler and an engine cylinder due to the pipe 
line. It is in order to make up this "drop" that the field magnet 
is compound-wound. (See Q. 138.) 

Q. 201 — How much drop is usually allowed in wires ? 

A. — From 2 per cent to 10 per cent, according to the conditions. 
In the wiring within an ordinary building the drop is usually 2 per 
cent of the maximum E.M.F. of the work circuit. In mills and 
factories the drop in the mains is ij to 2 per cent, and the total 



Constant-Potential Circuits, 



69 



drop is sometimes as high as 10 per cent when water power is 
used to drive the dynamo. 

Q. 202 — What are mains ? 

A. — The wires from which the lamps, etc., are directly fed. 
Fig. 90 shows a diagram of a simple circuit consisting of one 
feeder, one main and several taps, marked t. The circles repre- 
sent lamps. 

Q. 203^Why are not the lamps, a, b and c, connected directly 
to the feeder ? 

A. — Because it is desirable that the voltage shall be as near the 

fOi 



rOi 



A 



i—4 



MAIN 



t-^ 



X 



A 



MAIN 



t 1 I ■ It ) 



^ ^-^ 



^ * 



Power, N. Y. 




FIG. 90. 

same at all the lamps as possible, and the voltage of the feeder 
at X would be higher than that of the main. Moreover, it would 
confuse the general plan to tap from a feeder, although it is some- 
times done. Strictly, a feeder should never be connected with 
anything between its extremities. 

Q. 204 — Is there not a drop in voltage in the main ? 

A. — Yes ; the E.M.F. is highest where the feeder connects and 
lowest at the lamp furthest from this junction, but the difference 
is not over 2 volts and sometimes only a fraction of a volt. 



70 



Constant-Potential Circuits. 



Q. 205 — Does a dynamo supply more than one circuit ? 

A. — A constant-potential dynamo almost always does. A 
series-wound, or constant-current dynamo sometimes does. Fig. 
91 shows three feeders supplied from one constant-potential dy- 
namo. Fig. 92 shows two circuits supplied from one constant- 
current dynamo. 



mro 




B 



FIG. 91. 



Q. 206 — In Fig. 91 why could not a single feeder be used for 
all three mains? 

A. — It could be done and occasionally is, but three feeders give 
better distribution and better control of the circuits. Any one of 
the three circuits can be cut off at the dynamo without affecting 



C onstjint-Current Circuit — Wire Sizes. 



71 



the other two. If only one feeder served all three mains, the dif- 
ferent groups could not be controlled from the dynamo room. 

Q. 207 — How is the drop in a feeder determined ? 

A. — By the load, the size of the wire and the feeder length from 
the dynamo to the main. The drop may be calculated by the fol- 
lowing formula, in which D represents the distance in feet from 
end to end of the feeder ;'^ C, the current it has to carry ; A, the 



y-^ — y — 


— X— 


— K— 




"^ 














' 


( 












J 




^ 










J 


• 










'"• 










■~N 


j^ 












c 


V ; 














{ 












)> ^ 


{ 










> f 












^ > 


( 


-x^ 


— ^?<— 






, I 




^ — )f— 


X 


— X— 


:f 








;t 


/■ ^^ 

















FIG. 92. 



cross-sectional area of the wire in circular mils, and v, the drop in 
volts : 

21XC XD 

-2 = "'' (24) 

This formula may be used to find the size of wire necessary for 

a given load, distance and drop by transposing it into 

21 XC X D ^ 

— =A (25) 

V 

Example. — A feeder 700 feet long from end to end must carry 



♦The actual length of one leg of the feeder. 



72 



Wire Sizes and Capacities. 



a load of 270 amperes with a drop of 2 volts; what is its area 
in circular mils? 

C = 270 ; D = 700, and v = 2. 

21 X C X D 21 X 270 X 700 

A — = 

V 2 

which gives 19,845 circular mils as the area. In Table III of 



Table III 









Ih 


<u u 


Safe Carrying 




il 






a 3 


Capacity in 







i> 


cSSfe 


Amp 


eres. 


^ 


3^- 


-5 










E 






3 


.£ = 




CQ-H 


°dco 


"O 




V 


(U 


S3 


-S 


3*0 '^ 


0) 


T) ^ 


to 

3 



Q 


.2 " 


bJC.- 




V 


3 « 

vt in 


B&S 


d 


d2=C.M. 


lbs. 


1000 ft. 
in Ohms. 


n 




0000 


460 


211600 


640.5 


.050756 


620 


210 


000 


410 


167805 


508.0 


.064"04 


525 


177 


00 


365 


133079 


402.8 


.080704 


438 


150 





325 


105593 


319.6 


.101712 


369 


127 


1 


289 


88694 


253.4 


.128318 


309 


107 


2 


258 


66373 


201.0 


.161812 


260 


90 


3 


229 


52633 


159.3 


.204040 


219 


76 


4 


204 


41743 


126.4 


.257291 


183 


65 


5 


182 


33102 


100.2 


.324441 


154 


54 


6 


162 


26251 


79.46 


.409136 


130 


46 


7 


144 


20817 


63.01 


.515('82 


109 


38 


8 


128 


16510 


49.98 


.650526 


92 


33 


9 


114 


18094 


39 64 


.820222 


77 


28 


10 


102 


10383 


31.43 


1.0345,'iO 


65 


24 . 


11 


91 


8234 


24.93 


1.304340 


54 


20 


18 


81 


6530 


19.77 


1.744740 


46 


17 


13 


?'3 


5178 


15.68 


2.n740no 


38 


14 


14 


64 


4107 


12.43 


2.680440 


32 


12 


15 


57 


3257 


9.86 


3.297820 


27 


9 


16 


51 


2583 


7.82 


4.15812U 


23 


6 


17 


45 


2048 


6.20 


5.243630 


19 


5.4 


18 


40 


1624 


4 92 


1.612080 


16 


3 


Mil. 


1 


1 


.003027149 


10740. 


.0629 


... 



wire constants, the nearest size to this is No. 7 wire, with an area 
of 20,817 circular mils; this size would be used. 

Q. 208 — Are feeders for three-wire circuits figured out like 
two-wire feeders? 

A. — The outside wires are. The size of the neutral wire is 
figured on the basis of the maximum difference which may exist 



Three-Wire System. 73 

between the loads carried by the two sides of the system under 
the most extreme probable conditions.* 

Q. 209 — Why is the three-wire system used instead of the 
simpler two-wire arrangement? 

A. — Because of the greater economy in the cost of the wire. 
For large plants the conductors must be very large and expensive. 
Using the three-wire system enables one to use double the 
voltage, therefore, one-half the current, and, consequently, smaller 
wires for a given amount of work. For example, if we have 
1000 lamps to maintain, of 50 watts each, the total load will 
be 50,000 watts, or 50 kilowatts. With the two-wire system, even 
at 125 volts instead of the usual no, the total current will be 
50,000 -f- 125 = 400 amperes. (W = E X C; hence, C = W 
■i- E). Suppose one feeder 500 feet long carried the load, with 
I -J volts drop, its area would be 280,000 circular mils, according 
to the wiring formula. Now, with three wires, assuming the 
load to be equally divided, half of the lamps would be in series 
with the other half, so that the current would be one-half as great 
as before, or 200 amperes, and the voltage from outside to out- 
side would be 250. If a loss of i J volts was allowable at 125 volts, 
3 volts is allowable at 250 volts to give the same proportion (per- 
centage) of loss. Then the area of the two outside wires 
would be 

21 X 200 X 500 

= 70,000 circular mils, 

3 

or just one-fourth of the area (and, therefore, the weight) re- 
quired for the two-wire system. The middle or neutral wire 
would be given an area about one-half that of each outside wire, 
so that, in the case assumed, the total area of feed wires would be 
70,000 + 70,000 + 35,000 = 175,000 circular mils for three 
wires, against 280,000 + 280,000 = 560,000 circular mils for two 
wires, with the same load and loss. Accurately, the difference 
would be not quite so great owing to the fact that the load is very 
seldom divided exactly equally. Under average conditions the 
feeders and mains will have about one-third the total area for 
three-wire distribution that they would have for two wires, the 
load and drop being the same in both systems. 

t See Poole's "Electric Wiring" for fuller information. 



74 



Maxiinuni Safe Current. 



Q. 210 — What is meant by circular mils? 

A. — A mil is ^^^ of an inch, so that a wire \ inch in diameter 
would be 250 mils in diameter. The area in circular mils is found 
by "squaring" the diameter, or multiplying it by itself. It is 
sometimes written d'~ and sometimes "circ. mil." 



f'^f 



f'Of' 



f f 



o 

z 







// 



Power, N.T. 



FIG. 93. 



f f'f'r 



f'f'f'f 



Q. 211 — What determines the "maximum load allowed," speci- 
fied in the table? 

A. — The maximum allowable load is fixed by the National 
Board of Fire Underwriters. Passing current through a wire 
heats it, and the more current the greater the heat; the figures 
given represent the greatest loads the wires will carry without 
heating above a safe temperature. 

Q. 212 — What limits the amount of current in a wire? 



Safety Fuses. 



75 



A. — The resistance of the complete circuit, of which it forms 
a part. So long as things are normal the amount of current does 
not exceed the value intended. Should the opposite legs of a main 
or feeder or tap become ''short-circuited," i. e., connected by a 
wire of negligible resistance, or brought directly together, an ab- 
normal current would flow. To prevent such a current from con- 
tinuing long enough to damage the wiring and dynamo, every 
circuit is provided with a safety device which breaks the connec- 
tion with the dynamo, and thus cuts off the current, when it be- 
comes too great. 

Q. 213 — What is this safety device like? 

A. — Most safety devices are fuses ; sometimes a mechanical de- 




FIG. 94. 

vice known as a circuit-breaker is used. A fuse is simply a wire 
or flat strip of soft metal which melts before the current reaches 
a point dangerous to other parts of the circuits. 

Q. 214 — Is a fuse used on each circuit? 

A. — Several fuses are used on each circuit. A fuse is inserted 
in each leg of the circuit at every point where there is a branch, or 
where the size of the wire changes. In Fig. 93, for example, a 
fuse would be located at each point marked F, f and f^; and each 
fuse would be of a different size from the others, excepting that 
all of the f^ fuses would be alike. 

Q. 215 — What determines the size of a fuse? 

A. — The maximum safe carrying capacity of that part of the 



76 



Fuse Blocks. 



circuit beyond the fuse and up to the next fuse. In Fig. 93, the 
fuses, F, in the feeder, near the dynamo, would be capable of car- 
rying the maximum safe current allowed for the size of wire used 
in that feeder, and the fuses at the far ends would be capable of 
carrying the safe current allowed for the size of wire used in the 
main, and so on. When only one dynamo is used, however, the 
fuse nearest the dynamo must not be large enough to pass a 
greater current than the dynamo can safely stand, no matter how 
great the capacity of the feeder may be. 

A. — Each dynamo is connected, by "leads," to the bus-bars, and 
the feeders are led out from these bus-bars. Fuses are inserted 
between the dynamos and the bus-bars and between the bars and 
the feeders, as in Fig. 94, the fuses, F, in this case being pro- 




FiG. 95. 



portioned for the dynamo capacity, regardless of the wiring, and 
all others being proportioned for the wire capacity. 

Q. 217 — How are the fuses connected to the wires? 

A. — They are mounted on porcelain or slate insulating bases 
between heavy brass terminal pieces, to which the circuit wires are 
attached. The complete arrangement is known as a fuse-block, 
or cut-out. Fig. 95 shows the simplest and smallest form of 
fuse-block or cut-out, known as a "bug" cut-out. The ends, L 
and Rj of the wire are fastened in place by means of set screws, 
L^ and R^, shown in the cut. Those binding screws press to- 
gether the two sides of a brass clamp, and hold the wire. In the 
cover of the cut-out at A, A, are two plates, also of brass, which 
fit in under the clamps when the cover is in place, as shown by 



Fuse Blocks, 



77 



the dotted line. These plates are isolated from each other, except 
for the link of fuse-wire, F. Now when the cover is on, the cur- 
rent can pass along from R^ to L^, through F. F is made in ac- 
cordance with the answer to Q. 215, and therefore there is no 
danger of overheating the circuit wire. The cut-out is "single- 
pole,'' meaning that it contains only a single fuse. Hence, two of 
them must be used to protect a circuit ; one in each leg of the cir- 
cuit. It is used for very light circuits, such as wall brackets, 
carrying two or three lamps. 

Q. 218 — What are cut-outs of larger size like? 

A. — In larger cut-outs both fuses are mounted on a single block, 




*«>•, iKT; 



FIG. 96. 



as in Fig. 96, which shows a "main line" covered cut-out or fuse- 
block. In this case, also, the fuses (F and F'^) are in the cover, 
and the manner in which they make the connections should be ap- 
parent from the sketch. All of the white portion is porcelain or 
some other non-conductor, and the dark portions are metal. The 
main wires being cut, the ends thus formed are run into the holes 
at the ends, as shown, and fastened in place by means of the 
binding-screws, S, S, and S^, S^. The other screws are used to 
secure the cover to the base, and at the same time secure good con- 
nection between the ends of the wires and the fuse clips. For 
branches or taps, at right angles to a main, the style of block 



78 



Fuse Blocks. 



shown by Fig. 97 is used ; this is known as a "branch block." The 
principle is the same as in the last case, but the main wires are 
not cut at all. They (L and L^) are led through the channels 
shown in the base, and secured to the binding posts, 6^ and vS"^. 
The branch wires, B and B^, terminate at the holes in the end of 
the cut-out base, where they are secured by the inner binding 
screws. The fuses are at F and F^. The dark portions being con- 
ductors and the light parts being non-conductors, it will be seen 
that as far as the branch circuit is concerned, the result obtained is 
the same as if the wires were connected direct, as in Fig. 90. 
Q. 219 — How are the fuses arranged in a series circuit? 




^.y 



FIG. 97. 



A. — Fuses are not used on constant-current systems. They are 
not needed, because the current is maintained practically at one 
value all the time, either by the dynamo itself or by an automatic 
regulator. 

Q. 220 — When a fuse melts what is done? 

A. — The circuit is disconnected from the system and a new fuse 
is put in the block. Then the circuit is re-connected. If the fuse 
blows (melts) again, the trouble on the circuit must be removed 
before the circuit is reconnected. 

Q. 221 — How are the wires placed in a building? 

A. — Sometimes on the surface of the ceiling and walls, but more 
generally between floor and ceiling and inside the walls. The two 



Classes of Wiring. 



79 



classes of wiring are known as surface wiring and concealed wir- 
ing. Surface wiring is divided into two sub-classes, namely, cleat 
wiring and molding work. 

Q. 222 — What is cleat wiring? 




FIG. 98. 



A. — An installation in which the wires are fastened to the walls 
and ceilings at intervals by cleats, as in Fig. 98, which shows a 
little stretch of three-wire mains, from each side of which two- wire 




FIG. 99. 




Power, N. V. 



FIG. 100. 





FIG. 102. 



taps or branches are led. Where one wire crosses another an extra 
insulation must be used, as at A, A. This may be a piece of rub- 
ber tubing or a heavy coating of tape. Figs. 99 and 100 show a 
two-wire and a three-wire cleat, respectively. 



8o 



Molding and Conduit Wire. 



Q. 223 — What is molding work? 

A. — Wiring placed in grooved molding. Fig. loi is an end 
view of one form of two-wire molding for ceilings and side walls, 
and Fig. 102 is a form of side-wall molding. Fig. 103 shows a 




Poivmr, N.r. 



FIG. 103. 



three-wire molding corresponding with Fig. loi. These mold- 
ings come in long strips. There are two parts, the "backing" con- 
taining grooves for wires, and the "capping," used to cover the 
grooves. The backing is fastened to the ceilings, walls, or floors 
by countersunk screws, as shown in the figures, and the capping 
is fastened to the backing by means of round-headed brass screws. 
Q. 224 — How is concealed wiring arranged? 







FIG. 104. 



A. — The wires are sometimes secured to porcelain knobs, where 
they run parallel with joists and beams, and run in porcelain tubes 
through the beams when the latter are of wood and at right angles 
to the wires. In most modern buildings, however, the wires are 



Choice of Wiring System. 



8i 



run in tubes of insulating material, or metal tubes with insulating 
linings, some rigid and some flexible; sections of both kinds are 
shown in Fig. 104. Such a system is called conduit wiring. 

Q. 225 — What are the conditions determining the system of 
wiring to use ? 

A. — Cleat wiring is used wherever appearances make no differ- 




FIG. 105. 

ence and the surrounding atmosphere is thoroughly dry, as in 
most mills, factories, plain storerooms, etc. It is the cheapest 
to install and easiest of access. Molding work comes next in 
point of cost and easy access, and is used where the molding is 
not considered disfiguring and the surroundings are dry. It must 
not be used in damp places because the wood absorbs moisture 
and leakage is liable to result. In finely finished buildings con- 



82 



Concealed Wiring. 



cealed wiring is always used ; if the building has ?. wood frame the 
wiring is sometimes put on porcelain, but the conduit system is 
much preferable because of the greater protection to the wires, and 
because the conduits can be put in while the building is being 




FIG. 106. 

erected and the wires can be drawn through afterward. In iron 
frame buildings the conduit system is imperative if the wiring is 
to be concealed. 

Q. 226 — How are fuses replaced in concealed wiring, if they 
blow ? 




FIG. 107. 



A. — The lamps are divided up into little groups, each having a 
separate feeder. All the feeders run from one point, where the 
fuses are located on a composite fuse-block called a tablet board. 
This is put in a little closet set in the wall. The switches for the 



Szvitches. 



83 



feeders are usually put in the same closet. Fig. 105 shows four 
groups of lights fed from a tablet board, and Fig. 106 is a typical 
distributing closet with tablet board and main switch. 

Q. 227 — Are not branch circuits ever used in concealed work? 




FIG. 108. 



FIG. 109. 



A. — When the wiring for a small load is distributed over a large 
area branch circuits are put in. In such a case a "junction box" 
is set in the wall, like the closet, and the ends of the branches lead 
into it. Fig. 107 show^s a "junction box" and its tablet block. The 
holes in the sides of the box have projecting flanges to which the 
ends of insulating tubes are fitted w^atertight. The main wires are 
bared and held under the large screws in the center of the fuse 
block and the ends of the branch wires are held under the smaller 
screws. Fuses go across between the pairs of very small screws 
and are separated by ribs of porcelain on the base. 

Q. 228 — How is a circuit connected and disconnected? 

A. — By means of switches. Fig. 108 shows a switch for break- 
ing a single conductor ; Fig. 109 shows another form designed for 
heavier currents, and Fig. no is a diagram of the connections. 

Q. 229 — Why is the switch in Fig. 109 better for heavy currents, 
than that in Fig. 108? 4 

A. — Because the contact between the lever and the terminal has 




FIG. no. 



more area and the parts can be pressed more firmly together. 

Q. 230 — Have these switches distinguishing names ? 

A. — Yes. Fig. 108 shows a single-pole button switch and Fig. 
109 a single-pole knife switch. Both of these are single-break, 



84 



Double-Break Switch. 



meaning that the circuit is broken at one point only. Fig., iii 
shows a single-pole double-break switch ; the diagram for this is 
shown by Fig. 112. 

Q. 231 — What is the object of having a double break? 




A. — In order to divide the flash into two parts. When a circuit 
is opened the current follows across the break until the opening is 
so wide that the E.M.F. will not force the current across. If the 
circuit is opened at two points in series at the same instant, the 
E.M.F. is divided between the two breaks and the length to which 
it will maintain a flash at either break is reduced to one-half. 
Another reason for providing two breaks is to avoid using the 
switch-blade pivots as conductors, the contact between the pivots 
and the blades being too loose for good conductivity. 



!? t 



FIG. 112. 



Q. 232 — Are switches ever made with more than two breaks ? 

A. — No ; a multiplicity of breaks would complicate the construc- 
tion tremendously. Moreover, it is almost impossible to make 
several contacts separate at precisely the same instant, and unless 



Douhle-Pole Switches 



8s 



they did so the value of a multiple break would be largely re- 
duced. For very heavy circuits "quick-break" switches are used. 
Fig. 113 represents a single-break switch of this type. The con- 
tact blade is held between the jaws by their clamping friction until 




FIG. 114. 



the handle compresses the spring sufficiently to force the blade 
out; as soon as it leaves the jaws, the spring expands and drives 
it away from the jaws with far greater rapidity than the hand 




FIG. 115. 



FIG. 116. 



could possibly do it, thus reducing the duration of the flash to a 
very small fraction of time. 

Q. 233 — Are switches put in both legs of a circuit, like fuses? 

A. — Yes ; double-pole switches are universally used for two- 



(S^r^r. 



LP o 




Q o- 



FIG. 117. 



wire constant-potential circuits. Fig. 114 illustrates the single- 
break double-pole form, of which Fig. 115 is the diagram. Fig. 
116 is the double-break, double-pole switch, of which Fig. 117 is 
For three-wire circuits, three-pole switches like 



the diagram 



86 



Triple-Pole Szvitch. 



Fig. ii8 are used; Fig. 119 is the corresponding diagram. These 
are also made with a double-break in each poh or leg, like the 
single-pole and double-pole switches already shown. 
Q. 234 — What are the thumbscrews on the base for? 




FIG. lis. 

A. — To hold fuses ; this switch is a combined switch and fuse- 
block on a single base. This form is frequently used in small 
plants. 

Q. 235 — Are there any other kinds of switches? 

A. — There is one more general class, known as the double-throw 




d 



switch. Whenever it is desirable to open one circuit and imme- 
diately close another, or to transfer one or more connections from 
one circuit to another in the least practical interval of time, a 
double-throw switch is used. Also, when one connection is to be 



Dottble-Throz^' Switches. 



87 



broken and another closed, and it is undesirable to allow both to be 
closed at the same time, a double-throw switch is used. Fig. 120 
illustrates the single-pole double-break form, and Fig. 121 is its 
diagram of connections. Fig. 122 is the double-pole single-break 




FIG. 324. 



O ^ Power. N.r. 

FIG. 125. 



form, the diagram of which is shown by Fig. 123. Fig. 124 is the 
triple-pole, double-break, double-throw switch; Fig. 125 is the 
-corresponding diagram. The last two are for very heavy cur- 
rents, and their blades are therefore made up of several strips to 
give greater contact area in the jaws. 



88 



The Switchboard. 



Q. 236 — How are the switches and fuse-blocks arranged in a 
dynamo room? 

A. — They are all grouped on marble or slate slabs, held in a 
vertical position either by braces from the wall or by legs, or both. 
The complete arrangement is called a switchboard. Fig. 126 
shows a small switchboard for two dynamos. Beginning at the 
top of the board, there are three indicating instruments ; the cen- 
tral one is a voltmeter, to show what the E.M.F. is and the other 
two are ampere-meters, or ammeters, each indicating the output 




FIG. 126. 



of its dynamo in amperes. Immediately below the instruments is 
a row of feeder switches which serve to connect and disconnect the 
various feeders with and from the bus-bars, which are mounted 
behind the board. Next comes the name-plate in the center, and 
on each side of its lower edge are the rheostat hand- wheels; the 
rheostats are behind the switchboard and the hand-wheel stems 
pass through it. The two large switches near the edges of the 
switchboard are the dynamo switches, connecting the dynamos 
with the bus-bars. At vs, below the name-plate, is a knob which 
works a rotary switch on the back of the switchboard. This 



The Circuit-Breaker. 



89 



switch is called the voltmeter switch because it serves to connect 
the voltmeter with various parts of the system, so that the E.M.F. 
at any important point may be instantly ascertained by the at- 
tendant. Immediately below the voltmeter switch is a double- 
throw switch, the function of whic'' is to transfer the bus-bars 
from connection with the dynamo switches to one with some other 
source of current, such as a street circuit, in the event of a failure 
of the dynamo or engine. At the bottom are two automatic 
switches, called circuit breakers. 

Q. 237 — What are circuit breakers? 

A. — Mechanical devices, used instead of fuses, to open a circuit 
when the current goes beyond a certain value. The usual form is 
a knife-blade switch (either single-pole or double-pole, according 




FIG. 121 



to requirements) normally held shut by a latch and provided with 
a strong spring which tends to open the switch. A magnet is also 
provided which releases the latch and allows the spring to throw 
the switch open as soon as the current reaches the point for which 
the magnet and latch are adjusted. Fig. 127 shows a double-pole 
circuit breaker in which two separate knife-blade switches are 
used. The more common form has the blades coupled together 
by an insulating cross-bar, exactly like a hand switch. The mag- 
net is a solenoid consisting of a few turns of heavy wire; its 
plunger trips the latch. 

Q. 238 — Why are circuit breakers used instead of fuses? 

A. — For two reasons. First, because they are more sensitive, 
and can be adjusted to open precisely at a given current value, 



90 



Switchboard Connections. 



just as a safety valve opens at a given steam pressure. Secondly, 
because it requires much less time to reset a circuit breaker than 
to replace a fuse. 

Q. 239 — Why are not circuit breakers used entirely in place of 
large fuses? 

A. — In some plants they are ; no fuses are used on any part of 
the switchboard. The only reason they are not always substituted 
for large fuses is that they are vastly more expensive in first cost. 
For the same reason and also because small fuses are more re- 
liable and less objectionable than large ones, circuit breakers are 
scarcely ever used instead of small fuses. 






EQUALIZER BAR 



BUS BAR 



BUS BAR 



■4-4-^ 



BUS BAR 



[l^n 



DYNAMO 
SWITCH 

a a 



TO DYNAMO A 



^4^ 



i — f 



Power, N. Y. 



nz] 



-CJ 



[ZZt- 



^ 



i:ij 



DOUBLE THROW SWITCH 



FIG. 128. 



n 



D 

□ □ D 



DYNAMO 
SWITCH 



TO STREET 
CIRCUIT 



TO DYNAMO B 



Q. 240 — In Fig. 126 why are the dynamo switches triple-pole? 

A. — The dynamos are operated in parallel, and the middle blade 
connects the equalizer wire, E, Fig. 88, with the equalizer bar. 
See also Fig. 128. 

Q. 241 — Why are triple-pole feeder switches used? 

A. — Because the wiring is laid out on the three-wire plan. This 
switchboard is the type used for large buildings having their own 
electric plants. In such buildings the wiring is customarily put in 
on the three-wire plan, so that when necessary it may be thrown 
on to the Edison three-wire street circuit. When supplied from 
the house dynamos, however, it is connected on the two- wire plan 
by connecting the outside legs together and putting the two sides 



Voltmeter Switch. 



91 



of the system in parallel. The double-throw switch just above 
the circuit breakers accomplishes this, as shown by the diagram, 
Fig. 128. 

Q. 242 — If two dynamos are to be used why are they not con- 
nected on the three-wire plan to save wire ? 

A. Because the load is so small during about three-fourths of 
each day that only one dynamo is necessary, the two being run to- 
gether only during the period of heavy load, which does not exceed 
iive or six hours a day. 




TO DYNAMO B 



TO EXTREME 
END OF 
CiRCUIT-f- 



Power, N. Y. \ 



TO CENTER 
OF DISTRIBUTION 



TO DYNAMO A 

+ 



TO BUS BAR + 




TO DYNAMO A 



TO DYNAMO B 



TO CENTER 
OF DISTRIBUTION 

FIG. 129. 



Q. 243 — How is the voltmeter switch arranged and why is it 
necessary ? 

A. — The connections are as shown by Fig. 129, from which it 
is evident that the voltmeter can be connected with the terminals 
of either dynamo, or with the bus-bars, or with either a central 
point or remote point in the lamp circuits. Under ordinary con- 
ditions it remains connected to the circuit at the central point of 
distribution. When one dynamo is already in circuit, however. 
and it becomes necessary to connect up the other one, it is impera- 
tive that the E.M.F. of the added machine shall be exactly the 
same as the E.M.F. at the bus-bars. Hence, the connections to 
dynamo terminals and bus-bars, which enable the attendant to 



92 



Central Station Szvitchhoard. 



compare the voltage at both before closing the dynamo switch. 
All the + connections are on one side of the circle swept by the 
switch and all the — connections are on the other side. 

Q. 244 — Why must the E.M.F. of the added dynamo be exactly 
the same as the bus-bar voltage before it is thrown in? 

A. — To avoid forcing current backwards through one of the 
dynamos. If dynamo A, Fig. 83, is supplying the circuit, the bus- 
bar E.M.F. being 120 volts, and dynamo 5 is thrown in circuit with 
an E.M.F. of 115 volts, dynamo A, being 5 volts stronger, will 



POSITIVE BUS B\R 



U^ 



IX [ [ [e^ua[lize!r 



! r I I 

N EGA];iVE|^ BUS [^3AR [^ [^ 



i i 

SWITC-' 




TO DYNAMO A TO DYNAMO B TO DYNAMO C TO DYNAMO D EARTH 



FIG. 130. 



TO FEEDERS 



force current backward through dynamo B. The result will be 
the same as though dynamo B were lying idle and subjected to an 
E.M.F. of 5 volts, generating none in opposition. If the resistance 
of the two dynamo armatures and the intermediate connections 
were 4_ of an ohm, the current flowing between them would be 
E 120 



^=R 



0.04 



= 3000 amperes. 



Q- 245 — Would not such a heavy current throw the circuit 
breakers and protect the dynamos ? 

A. — The circuit breakers would be thrown, but in the brief in- 



Central Station Switchboard. 



9S 



stant required for the magnet to act a heavy strain would be in- 
flicted upon both dynamos, which might result in damage, 

Q. 246 — How does the switchboard shown in Fig. 126 differ 
from one used in a central station. 

A. — In the arrangement for transferring the circuits to some 
other source of current and the manner of working the dynamos. 
In a central station the dynamos would most likely be operated on 
the three-wire plan, and if more than two were used they would be 




TO 250-VOLT DYNAMOS 
.Power, N. y. 

FIG. 131. 



TO 125- VOLT 
DYNAMO 



arranged to work in parallel on each side of the system. Fig. 130 
is a diagram of switchboard connections for a three-wire central 
station where four dynamos, all of equal size and voltage, are used. 
The equalizing wires are marked E. The voltmeter and its 
switch are omitted. 

Q. 247 — Why are double-throw dynamo switches used? 

A. — So that each dynamo may be connected to either side of the 



94 rhe Lightning Arrester. 

system. When the switch is left in its middle position the dynamo 
is disconnected entirely. 

Q. 248 — Why are so many ammeters necessary? 

A. — The individual ammeters are needed to show the load on 
each dynamo. The ammeters marked -\- and — show the total 
output on each side of the system. 

Q. 249 — Are three- wire systems always arranged as shown in 
Fig. 130? 

A. — No; an arrangement sometimes used is shown by Fig. 131. 
The main dynamos are 250-volt machines, and any difference be- 
tween the loads on the two sides of the system is taken care of by 
a smaller 125-volt dynamo, which can be put on either side of the 
neutral wire, as occasion requires. 

Q. 250 — In Fig. 130 the feeders are represented as passing out 
through lightning arresters. What are they? 

A. — A lightning arrester serves to prevent the lightning from 
damaging the dynamos and other apparatus by diverting it from 
the circuit wires into the earth — ''side-tracked" it, so to speak. 
Lightning nearly always takes the nearest path (electrically) to 
the earth, and by providing a side path from the circuit directly 
to the earth it is drawn off. 

Q. 251 — Does not the side path connect the circuit to the earth ? 

A. — No. Fig. 132, which illustrates the principle of lightning 
arresters, will show why. A toothed brass plate, c, is connected 
to the circuit wire and another one, d, just like it. is connected to 
the ground. The gaps between the teeth of the plates are very 
small, and while the regular working current will not jump 
across, lightning, which is electricity of enormous E.M.F., will do 
so readily. Hence, when lightning comes in on the line wire, A, 
as soon as it reaches the arrester it jumps over to the ground 
terminal, as d is called, and goes to the earth. In order to make 
the path as good as possible, a large metal plate, G, is buried in 
the ground deep enough to be in moist earth and connected to the 
ground terminal, d. 

Q. 252 — Is this arrester used in electric light and power sta- 
tions ? 

A. — No ; it is only used in telegraph and telephone work where 
very weak currents are employed. It forms the basis of all gap 
lightning arresters, however, and for this reason is described here. 



Electro-Mechanical Arrester. 



95 



The heavy duty arresters are based on the same principle, but the 
construction is different. When heavy currents are used the dy- 
namo current will follow the lightning across the gap and estab- 
lish an ''arc'' or continuous flame from one plate to the other, de- 



LINE 



FIG. 132. 

stroying the plates and causing other more serious damage. Hence, 
lightning arresters used on heavy duty circuits are designed to 
rupture the arc the instant it is formed. 

O. 253 — How is the arc ruptured ? 

A. — There are three general methods ; the oldest consists of 




Power, N. ¥. 



r;n. i';;; 



separating the plates or terminals of the arrester, the increased re- 
sistance thus obtained serving to break the flow of current across 
the gap. The next oldest is the magnetic blow-out and the latest 
type is called the "non-arcing" arrester, in which the terminals 



96 



Vapor and Electromagnetic Arresters. 



are made of a peculiar alloy that will not give off the vapor neces- 
sary to maintain an arc. Figs. 133 and 134 show two arresters of 
the first class. In Fig. 133 the ground terminal, d, is on the end 
of a long arm pivoted at p. When lightning passes across the 




i-uwer, y.r. 



FIG. 134. 



gap the dynamo current follows it and energizes the magnet, M, 
which instantly jerks the arm away from the line terminal and 
breaks the arc. The arm then falls back again, ready for the next 
flash. In Fig. 134 both of the terminals {e and /) are placed on 
pivoted arms, c and d, and project into the side of a small box, g. 




Power, N. Y, 



FIG. 135. 



When the arc is established between the points, e and f, which are 
of carbon, an intense heat results and an expansive vapor is liber- 
ated ; the hot air in the box expands, and, with the vapor, forces 
the terminals out of their holes and breaks the arc. 



Non-Arcing Arrester 



97 



Q. 254 — Why does not the flame burn off the ends of the car- 
bons? 

A. — It does, and on this account the arrester has to be adjusted 
frequently to keep the carbons near enough to each other. 

Q. 255 — How do the other arresters work? 

A. — The magnetic blow-out is illustrated by Fig. 135. Its 
action is similar to that of a strong air blast< A magnet will repel 
an electric arc, and in this arrester a powerful magnet, M, is ar- 
ranged with its poles close to the terminal plates, which curve 
apart, as shown. When the lightning draws the dynamo current 
across, the magnet drives the flame away from it to the extremities 
of the terminal plates, as at e, which are so far apart that the 



m 



J 



Power, N. Y. 



FIG. 136. 



current cannot continue across the space. This operation is prac- 
tically instantaneous, like blowing out a candle flame. The non- 
arcing arrester, Fig. 136, is simply a number of metal rods, c and 
d, mounted in blocks of insulating material, in and n, usually of 
porcelain. The end rods are connected to the two sides of the cir- 
cuit and the central one to the ground. There are thus several 
air gaps in series between the middle rod and each end rod. The 
lightning jumps across readily, but the dynamo current does not 
continue after the lightning discharge. This arrester protects 
both sides of a circuit ; the other types are single. It is used on 
alternating-current circuits only, but a non-arcing arrester for 



98 



Non-Arcinz Direct-Current Arrester. 



direct-current circuits, which is equally simple, is shown by 
Fig. 137. This consists of a base-board of lignum vitse, B, on 
which are mounted two metal blocks, c, d, constituting the ter- 
minals. Between the blocks are several very small grooves 
charred in the surface of the board ; the carbonized grooves fur- 
nish a path for the lightning, carbon being a conductor, but their 




Power, N. ¥. 

FIG. 137. 



resistance is too high to carry the dynamo current. The dynamo 
current is prevented from following a lightning discharge by 
shutting in the grooves with another piece of lignum vitse, the 
space then being so small that sufficient vapor to maintain an arc* 
cannot be contained. 



* The existence of an arc demands the presence of a vapor which reduces the re- 
sistance of the air gap. The vapor is created by the heat of the current volatiHzing the 
material of the terminals between which the arc is established. In Fig. 134 carbon 
vapor is produced. 



CHAPTER V. 

MEASURING INSTRUMENTS AND MEASUREMENTS. 

Q. 256 — What is the principle of the voltmeter ? 

A. — Some voltmeters are based on the magnetic needle prin- 
ciple, as shown by Figs. 138 and 138A. A soft iron needle, a, is 
pivoted within a coil of very fine wire, b, and held normally out 
of line with the axis of the coil by means of a permanent magnet. 
M, or a weight, zu. Passing a current through the coil, b, causes 
magnetic lines to flow (vertically in the illustrations) through 








iW 



Power, N.Y, 



FIG. 138. 



FIG. 138a. 



its center (see answer to Q. 99) and these tend to pull the needle 
around into line with them. The permanent magnet, M, or the 
weight, zv, resists this pull, and the distance that the needle is de- 
flected indicates the strength of the current in the coil. A pointer, 
e, is attached to the needle, a, and the scale, f, is marked in volts ; 
the pointer, e, indicates on the scale the E.M.F. at the terminals, 
c, d, of the coil, b. Fig. 139 shows the principle of a voltmeter 
in which a coil is pivoted between the poles of a magnet. Pass- 



er ^ 

..0( V. 



lOO 



Voltmeters. 



ing a current through the coil creates magnetic hnes of force at 
an angle to those supplied by the permanent magnet. These lines 
tend to pull the coil around so as to put themselves parallel with 
the ''field" lines of force. The coil is restrained by spiral springs, 




Power, N. Y. 

FIG. 139. 



which also serve as connections for the winding; one spring is 
below the coil, and omitted from the sketch. The external ap- 
pearance of Fig. 138 is shown by Fig. 140; that of Fig. 138A by 
Fig. 140A, and that of Fig. 139 by Fig. 141. 

Q. 257 — How does an ammeter work? 

A. — Exactly like a voltmeter. When the coil is stationary, how- 
ever, it is made of very heavy wire, because it is connected in 




Power, N. 7. 




FIG. 140. 



Power, N.Y. 

FIG. 140a. 



scries with the circuit and the full current goes through it. When 
the coil is pivoted it is made of fine wire, and a low-resistance 
shunt carries most of the current, but the dial is so graduated that 
the pointer indicates the whole current. 



Recording Watt-Hour Meters. 



lOI 



Q. 258. — What other measuring instruments are generally 
used? 

A. — Wattmeters and galvanometers. A wattmeter measures 




FIG. 141. 



the power furnished to a circuit or device. It is usually made in 
the form of a very small motor which drives a delicate train of 
gears, the hands of which indicate on dials the number of watt- 




FIG. 142. 



hours supplied. Such an instrument is really not a wattmeter 
but a watt-hour meter, because it registers the time as well as 
the power. Fig. 142 shows a widely used style of watt-hour 



I02 



Recording Watt-Hour Meter. 



meter. The two coils of wire enclose the armature and furnish 
the field magnetism. The fan blades on the lower part of the 
shaft serve as a brake and steady the rotation of the shaft. There 
are several forms of motor-meters, all based on the same principle 
—that of making the speed of the motor proportional to the watts 
supplied to the circuit. 

Q. 259 — How do these meters measure the time of service? 

A. — Simply by running when the current is passing and stop- 
ping when it stops. For example, if the motor shaft makes 10 
revolutions a minute when 100 watts are being supplied, at the 
expiration of an hour it will make 600 revolutions. If 200 watts 
go through it will make 20 revolutions a minute, or 1200 an hour, 
so that 200 watts for half an hour, or 100 watts for an hour or 50 




PQ-Olttl 



FIG. 143. 



watts for two hours will each cause the motor to make 600 revo- 
lutions total, and the registration on the dials will be the same in 
all three cases. Watt-hour meters are made for both direct and 
alternating currents. 

Q. 260 — Why is the speed of the motor proportional to ihe 
watts supplied? 

A. — Its field coils are in series with the circuit and no iron is 
used, so that the field strength varies exactly with the load. The 
armature is in shunt or parallel to the circuit, and its strength 
varies with the E.M.F. of the circuit. The product of the field 
and armature strengths, which varies with the product of the 
current and E.M.F., i. e., the watts, determines the speed. 

Q. 261 — What is a galvanometer and what is it used for? 

A. — A galvanometer is very much like the voltmeter in Fig. 



The Galvanometer. 



103 



140. It consists of a coil of fine wire in which is pivoted a hard 
steel bar magnet like a compass needle. (In a voltmeter the 
pivoted needle is of soft iron.) The galvanometer needle is in 
some cases held at zero by an outside magnet like the voltmeter 
needle, but in ordinary instruments it is simply allowed to point 
north and south like a compass needle, and the galvanometer is 
twisted around until the zero mark is under the pointer. The dial 
is graduated in degrees of the circle in some galvanometers ; in 
others it is not graduated at all, but simply has a zero mark. Fig. 
143 shows an ordinary galvanometer with a graduated dial. Gal- 
vanometers are used in testing for resistance, in an arrangement 
known as the Wheatstone bridge. 




Q. 262 — What is a Wheatstone bridge ? 

A. — A combination of resistance coils and a galvanometer, ar- 
ranged as shown diagrammatically by Fig. 144. Here A and D 
are coils of known resistance, 5 is a battery, G a galvanometer, F 
a series of resistance coils which can be cut in or out by a movable 
contact, c ; x, x, are binding posts to which the object to be tested 
is connected. Whenever the resistance of the test object (say a 
field magnet coil) X, bears the same relation to F that the resist- 
ance of D does to that of A, no current will pass through the 
galvanometer and its needle will remain at zero. Consequently, 
in using the apparatus the resistance, F, is adjusted until the 
galvanometer needle remains at zero, when the formula 

DF ^ , .. 
=X (26) 

A 



104 



The Wheatstone Bridge. 



gives the resistance of the test object. This formula is easily 
memorized by applying the "rule of three," thus 

A :D \ :F \X 
In many cases the resistances, A and D, are made equal ; then, of 
course, the resistance of F is the same as that of X when the gal- 
vanometer indicates zero. 




Q. 263 — Why does the galvanometer indicate zero when A : 
D \ :F '.X? 

A. — Because there is no difference of potential between ; and k 
to force current across the galvanometer. This can be demon- 
strated by Ohm's law. Suppose the resistances have the values 
given in Fig. 145, and a current of 4 amperes flows through D 
and X and i ampere goes through A and F. The "drop" in D 
from p to k will be 25 X 4 == 100 volts and the "drop" in A will 
be I X 100 = 100 volts, leaving the potential at / and k exactly 
equal. When the "balance" between the two sides is disturbed, 



p 



^^oO 




95 VOLTS ACROSS 
FIG. 146. 



current will flow from one side to the other. Figs. 146, 147 and 
148 illustrate this. In Fig. 146, a piece of wire, s, of negligible 
resistance is substituted for the galvanometer. This gives practi- 
cally the electrical condition shown by Fig. 147. In these two 
diagrams the bridge is not balanced, A being four times D, while 
F is three times X. The result is most easily figured by con- 



The Wheatstone Bridge. 



105 



sidering the conductances'^ of the four legs. The resistance of A 
being 100 ohms, its conductance is t^o; D has a conductance of 
^V. The conductance of the two, jointly, is xio + ^; or w + imy, 
which is rto. The resistance, therefore, of A and D combined, is 
^ = 20 ohms Similiarly, F and X have a combined conduct- 
ance of 3iro + rw or 3^0 + 3^0 = sU or yV; this means 75 ohms 




A = 100 



F = 300 



D = 25 -^ "^ -X = 100 

95 VOLTS ACROSS 

FIG. 147. 




Power, N.Y. 



resistance The total resistance from / to q, therefore, is 
20 + 75 r= 95 ohms, and the total current flowing will be i ampere. 
From p to 2 the current divides up into i ampere in A and i in D. 
from 2 to ^ it divides up into i ampere in F and | in X. Now in 
order to have \ ampere in A and \ in F, some current (2V ampere) 
must cross over from D to /^through the joint, Z. If the galva- 
nometer were inserted it would, of course, indicate the passage of 
this current and show that the bridge was ' ' out of balance ' ' 

Fig. 148 shows the bridge in balance, the ratio A: D being the 
same as F: X. The conductance from /> to ^ is tV + t^^% or 
7V + T5 = Ts; hence the resistance is ¥ or 15 ohms. The conduct- 
ance from 2' to ^ is T^o + t^o or tsu + tto = jf 0, so that the resist- 
ance is ~%- or 80 ohms, making a total resistance from p to q oi 95 



P 



A -75 



= 163 + 




95 VOLTS ACROSS 
FIG. 148. 



ohms, as before The total current will again be i ampere, but 
both A and F will carry I ampere while | flows through D and X, 
and no current will need to flow across at 2. 



* The conductance of a wire is equal to ^, or 1 divided by the resistance. If a wire 
has a resistance of 100 ohms its conductance is i^. The conductance of two or more 
wires connected in parallel is equal to the sum of their separate conductances, just as 
the resistances of several wires in series is the sum of their resistances. Inverting- the 
conductance gives the resistance, and vice versa. 



io6 Joint Resistance of Parallel Circuits. 

Q. 264 — Why is it necessary to compute both resistance and 
conductance ? 

A. — One computes conductance when considering parallel cir- 
cuits merely for convenience ; having the conductance, it is 
changed into resistance, because the relation between R, C and E, 
given by Ohm's law, which is the foundation of all wiring calcula- 
tions. The formula for the resistance of parallel circuits is 

Ra X Ri> 

R^l^b = "" '""'^'^ (^7J 

for two branches, Ra and Rb. For three branches it is worse, 
namely, 

Ra X Rb X Re 
RaXRb-hRbXRc + RcX Ra = ^ ^''^^^' ' ' ^^^^ 
It is evidently easier to compute 

RcT Rb' °^ Ra Rb Re 
and invert the result. 

Q. 265 — Is the Wheatstone bridge used for testing dynamos? 

A. — Sometimes, but it is more often used for testing objects of 
high resistance, such as small magnet coils and the insulation of 
circuit wiring. 

Q. 266 — How is insulation tested? 

A. — One way is to connect one of the x terminals of the bridge 
to one bus-bar, and the other x terminal to the other bus-bar ; turn 
all the lamps off at the sockets, but leave all circuit switches closed 
except the dynamo switches. The bridge then shows the resist- 
ance between the two sides of the circuit, and the amount of leak- 
age (usually too small to consider) may be ascertained. This ap- 
plies only to constant-potential or parallel circuits. 

Q. 267 — Why is the bridge not generally used for testing 
dynamos ? 

A. — Because the galvanometer is extremely sensitive and liable 
to disturbance from outside sources, such as heavy pieces of iron, 
dynamo magnets, etc. This trouble is sometimes evaded by using 
a telephone receiver instead of a galvanometer. The receiver is 
held to the ear and connected and disconnected by means of a 
contact key. As long as a clicking sound is distinguishable in the 
telephone when the key is opened and closed the bridge is out of 



Measuring Resistance by ''Drop." 



107 



balance; when no click can be heard, it is balanced, and the calcu- 
DXF 



lation 



A 



^ Xmay be made. 



Q. 268— Are there any other methods of measuring resistance? 

A. — Yes. One of the most convenient to use around power sta- 
tions is the ammeter and voltmeter method. This is represented 
by Fig. 149, in which x and x are the posts to which the test object 
is connected, and B, B are posts to be connected with a source of 
current, such as an incandescent circuit. The rheostat should be 



X 

Q 



X 

o 



B 

-o 



B 

O 




oOOOOq 



.0 



o 

o 
o 
o 
o 
o 
o 

o 

o 
o 




RHEOSTAT 



o 



O 



o 

o 
o 
o 



FIG. 149. 

capable of carrying 10 amperes continuously, and should have a 
total resistance of 100 to 125 ohms. The test is made by adjust- 
ing the rheostat until the ammeter indicates one ampere (if the test 
ob.iect can stand it), when the voltmeter will indicate directly the 
resistance in ohms of the object. If the latter is a heavy wire coil 
the rheostat may be adjusted until 10 amperes flow; then the re- 
sistance will be tV of the voltmeter reading. When iV ampere 
flows the resistance is ten times the voltmeter reading. The cur- 
rent supplied at the binding posts, B, B, must be "direct," not 
alternating. 



CHAPTER VL 
ALTERNATING CURRENTS. 

Q. 269 — What is alternating current? 

A. — A current, the polarity of which constantly changes from 
positive to negative and back again, i. e., which flows first in one 
direction, and so on. Referring again to Figs. 74 and 75, on 
page 54, it will be seen that in order to maintain a continuous 
flow of current in the circuit W it was necessary to introduce the 
commutator. Omit the commutator and make the connections 




ISlUJJJJJMAWJJXMJ 



w 



Power, N. Y. 



FIG. 150. 



from coil to circuit permanent by sliding contacts, as in Fig. 150, 
and the current in W will be alternating. When A passes the 5 
pole the current will flow from left to right in the outside circuit; 
when A passes the N pole the direction of the current is reversed 
throughout the whole circuit, flowing from right to left in W. 

Q. 270 — What are the rings called that the brushes bear upon. 

A. — Collector rings. No matter how many armature coils a 
simple alternator has, there are only two collector rings. 



Current Reversals. 



109 



Q. 271 — Does the current change from positive to negative 
suddenly ? 

A. — No; it rises from nothing or zero to maximum in one direc- 
tion (say positive), and then goes back to zero and rises to maxi- 
mum in the opposite direction; then it falls to zero and reverses 
again, and so on. When the loop or coil is midway between poles, 
as in Fig. 150, the E.M.F. is zero, and it rises gradually as the wire 
approaches the pole-piece, S, reaching maximum as the wire 
passes the center of the pole-piece, and then beginning to die 
away. 

Q. 272 — Can the variation in E.M.F. be shown by a diagram, 
as an indicator diagram shows the change in steam pressure? 

A. — Yes. Fig. 151 is such a diagram. If a drum Hke an indi- 

H 




FIG. 151. 



cator drum were revolved steadily and a pencil could be so ar- 
ranged as to rise above a central zero line as the E.M.F. rises in 
the positive direction and be drawn below the line as the E.M.F. 
rises in the negative direction, it would trace such a curve on the 
paper of the drum. 

Q. 273 — How does Fig. 151 apply to Fig. 150? 

A. — Comparison of Figs. 151 and 152 will show this. Fig. 152 
represents the four extreme positions occupied by any one wire of 
a coil during one complete pair of reversals. The E.M.F. at each 
point is shown by Fig. 151, the reference letters under the draw- 
ings of Fig. 152 corresponding with those on the diagram. The 
vertical dotted lines in Fig. 152 represent the passage of magnetic 
flux from pole to pole. 

Q. 274 — Then the vertical distance from the zero line to a point 
on the curve in Fig. 151 shows the E.M.F. at the point? 



no 



Effective Electromotive Force. 



A. — Exactly. The curve here shown is drawn to the scale of 
2,000 volts per inch. Hence the E.M.F. at H and / is 1,414.2 
volts, represented by the length of the lines EH and GI. 

Q. 275 — Why is no E.M.F. generated when the wire is mid- 
way between the poles? It is moving in magnetic lines. 

A. — Yes; but not cutting across them. The wire moves parallel 
with the lines for a short distance before it begins to cut across 
them, and parallel motion does not generate any E.M.F. 

Q. 276 — When the E.M.F. is always fluctuating, how can it be 
measured, and what is taken as the working E.M.F.? 

A. — The geometrical average is taken as the working E.M.F. 
This is called the effective electromotive force. It is measured 
by voltmeters designed especially for alternating current. The 




'i 1; ; 
S 

B& D 





FIG. 152. 



voltmeter can not respond to the enormously rapid fluctuations 
of the alternating current, but indicates the effective E.M.F. 

Q. 2yy — Is the effective E.M.F. one-half of the maximum? 

A. — No; it is 0.707 of the maximum. Or, to express it the 

other way, the maximum is 1.414 times the effective E.M.F. 

Thus, the effective E.M.F. of the curve Fig. 151 is 1,000 volts. 

The accurate relation between the two is 

E — 

— s^ = E; or E X >/2 = E . 

. > ■-' /^ -v max 

-v/2 

The square root of 2 is 1.4142136-1-, but in practical working 
1. 414 is considered sufficiently accurate; in fact 1.4 is frequently 
used as the ratio of E ^^^^^ to E, 

Q. 278 — Is the effective E.M.F., E, of alternating current the 
same as the E.M.F., E, of continuous current? 



The Cycle. ' iii 

A. — Yes, practically. It is assumed to give the same results, 
excepting the strain on insulation. When this is considered 
E is always used. 

Q. 279 — Then the E.M.F. tending to pass through the insula- 
tion of a looo-volt alternating-current dynamo is 1414 volts ? 

A — Of course; because the full E.M.F. exists periodically for a 
brief instant. In reckoning the circuit E.M.F. the effective pres- 
sure is considered, and whenever the terms voltage, potential, 
E.M.F. and pressure are used in dealing with alternating currents 
the effective E.M.F. is meant, unless the maximum or some other 
value is actually specified. 

Q. 280 — Does the E.]\I.F. always rise and fall twice in a revolu- 
tion as indicated by Figs. 151 and 152? 

A. — In a bipolar machine it would, because it rises and falls 
once every time a coil passes one magnet pole, or twice for each 
pair of poles. Alternators are always multipolar, however, so that 
the number of complete curves like Fig. 151, or "cycles," that are 
described in one revolution of the armature depends upon the 
number of poles. The num.ber of cycles per revolution is one-half 
the number of poles, 

Q. 281 — What is meant by cycles? 

A. — One rise E.M.F. in the positive direction and one in the 
negative direction, ending at zero, is a cycle. The number of 
cycles per second is called the "frequency." A physical concep- 
tion of an electrical cycle may be formed by remembering that 
one cycle of any series of repeated operations extends from the 
beginning of one operation to the point where that same opera- 
tion starts to repeat. For example, if one were engaged in a 
series of operations which were periodically repeated, such as 
carrying an armful of material from A to B, depositing it, going 
back to A empty handed, securing another armful, carrying it to 
B, and so on over and over, each complete set of operations up to 
the point where repetition begins, would be one cycle or "period," 
as it is also called. Hence, the passage of any one wire or coil 
or set of coils on an alternator armature from any given position 
with relation to the field magnet to the next identical position con- 
stitutes one cycle or period. For example, if a coil passes from a 
point opposite to the center of a north pole to a point opposite the 
center of a south pole, it will not have passed through a cycle br- 



112 The Cycle. 

cause the two poles are different. But when it reaches the center 
of the next pole the cycle is complete. 

Q. 282 — Then a cycle extends from the center of one magnet 
pole to the center of the next magnet pole of the same polarity? 

A. — A cycle doesn't "extend" in that sense. A wire traversing 
that distance passes through one cycle of generation. The cycle 
may be taken as starting at any point — not necessarily when a 
coil is under the center of a pole. The cycle generally considered 
is the one beginning when a coil is generating no E.M.F., as in 
the first and third positions shown by Fig. 152. The end of any 
cycle or period is reached when the coil or wire under considera- 
tion begins to do again precisely what it was doing at the be- 
ginning of the cycle or period. 



CHAPTER VII. 
ALTERNATING CURRENT GENERATORS. 

Q. 283 — Why are alternators always multipolar? 

A. — Because it is desirable for the frequency to be high — from 
30 to 140 cycles per second — and this would require too many 
revolutions of the armature for mechanical safety. For example, 
at 30 cycles, the lowest frequency generally used, the armature of 
a bipolar machine would have to make 30 revolutions a second, 




FIG. 153. 



or 1800 a minute, which is impracticable for machines of con- 
siderable output. 

Q. 284 — Are there dififerent types of alternators? 

A. — Yes ; there are three general types. The one most used 
consists of an ordinary multipolar field with a revolving armature,, 
as indicated by Fig. 153. A similar type has a stationary armature 
and a revolving field magnet. 



114 



Revolving Field Alternator. 



Q. 285 — How is the field magnet arranged to revolve? 

A. — It is mounted inside the armature, which is a ring of large 




FIG. 154. 




Power, N. T. ^ 



FIG. 155. 



diameter, as shown by Fig. 154. The field magnet is a wheel with 
poles projecting externally, as in Fig. 155, instead of internally. 



The Inductor Alternator. 



115 



Q. 286 — What are the advantages of such a type? 

A. — The generation of higher potentials, made practical by 
reason of the fact that the armature wires can be more securely 
fastened and more effectually insulated. The collector rings and 




FIG. 156. 



brushes are only two in number and carry small currents at low 
potentials, making them safe to handle. The armature terminals 
being stationary, they can be enclosed permanently so that no one 
can come in contact with them, and they can, therefore, have a po- 
tential much higher than would be safe for collector rings and 
brushes. 

Q. 287 — What other type of alternator is there ? 

A. — The inductor type, in which all of the wires are stationary. 
The principle is illustrated in Figs. 156 and 157, where a magnet 
M, is shown provided with extended poles, each divided into five 
small pole-pieces, the two rows facing each other, and wound with 
coils A, B, and C, D. Between the opposing rows of pole-pieces 
are as many iron blocks, E, as there are poles on each side. Now 
imagine the magnet M excited ; then one row of poles will be north 




FIG. 157. 



and the other south, as indicated, and lines of force will flow across 
from the A^ poles to the vS poles through the blocks E when they 
are in the position shown by Fig. 156. Now pull the blocks side- 
wise to the position in Fig. 157, and the number of magnetic lines 



ii6 



The Inductor Alternator. 



of force passing across through the coils is greatly decreased by 
reason of the higher reluctance* of their path. This decrease in- 
duces an E.M.F. in the coils A, B, and C, D, and if there were an 
endless string of blocks, E. and they were kept sliding, an impulse 




FIG. 158. 



of E.M.F. would be induced in one direction every time the mag- 
netism was increased by the relation shown in Fig. 156 and an 
impulse in the opposite direction would be induced every time 




■Power; ^_ 



FIG. 159. 



the position in Fig. 157 was occupied by the slider. Thus a 
regularly alternating current would flow in the coils if their ends 
were connected to a closed circuit. 



* Reluctance is magnetic resistance. 



The Inductor Alternator. 



117 



Q. 288 — How is this condition attained in an actual machine? 

A. — The blocks, E, are arranged on a drum, as in Fig. 158, and 
the pole-pieces on the interior of a shell, as in Fig. 159, and the 
drum is revolved within the shell so that the projections pass con- 




FIG. 160. 



tinuously before the pole-faces. The two rows of poles are side 
by side instead of facing each other, and there are two corres- 
ponding rows of projections on the revolving drum. The pole- 
pieces are magnetized by a single coil (Fig. 160) which fits in be- 




FIG. 161. 



tween them. Fig. 161 shows the shell with all the coils and Fig. 
162 the complete machine. 

Q. 289 — In this type of m,achine are the coils wound on the 
pole-pieces the armature coils ? 



ii8 



The Inductor Alternator 



A. — Yes; as the functions of the field and armature are so 
mixed up, however, the usual terms are not applied to an inductor 




FIG. 162. 



alternator. The stationary shell and poles as a whole are called 
the "stator ;" the revolving part is the "rotor," and is also called 
the "inductor" (an inappropriate and inexact name); the big 
single coil is called the field coil or the "exciting" coil; the smaller 




'ower,y.r. 



FIG. 163. 



coils are variously known as "induction" coils, "service" coils 
and armature coils. The last term is highly inappropriate and 



Alternator Armature Winding. 



119 



only used because these coils perform the same work that arma- 
ture coils do in a conventional type of dynamo. 

Q. 290 — Is the armature of an ordinary alternator wound like 
that of a continuous current dynamo ? 

A. — Not usually. In a simple alternator there are generally 
as many coils as magnet poles. A coil may have many turns of 
wire, but the number of coils is governed by the number of poles. 

Q. 291— Why? 

A. — Because the E.M,F. must rise and fall at practically the 
same instants in every wire on the armature in order that the 
individual E.M.F's of the wires may add up to the greatest total. 



l.N 




Fig. 163 shows an eight-pole alternator with eight armature coils, 
in outline. It will be noticed that every coil occupies exactly the 
same relative position with regard to the field, so that when the 
E.M.F. rises in one coil it rises in all of them. 

Q. 292 — Are the coils laid on the surface of the core, or in 
slots ? 

A. — Armatures are built both ways, but the slotted core is 
mostly used. 

Q. 293 — How are the coils connected? 

A. — All in series usually. Sometimes they are connected in 
two parallel groups, in which case the E.M.F. is one half as great 



I20 



Armature Winding. 



as when all are in series. Fig. 164 is a diagram of the series ar- 
rangement and Fig. 165 is a diagram of the other. As it is not 
practicable to show a complete cylindrical surface in a diagram 
the coils are drawn as though the armature were a flat disk and 




they were laid against one face of it. The poles are shown as 
though pointing parallel with the shaft, to correspond with the 
coil position. 




FIG. 166. 



Q. 294 — What would happen if another set of coils were put 
in the "blank spaces on an alternator armature core? 

A. — Each set would generate an E.M.F., but the E.M.F. of one 
would be maximum when that of the other was zero. Fig. 166 



Two-Phase Armature Winding. 



121 



shows why. The coils, m, m, are cutting magnetic lines at the 
maximum rate, while n, and n, are cutting none. The effect is the 
same, of course, in a multipolar machine like Fig. 167, where all 




Power, N.T. 



FIG. 167. 



of the coils shown as cutting lines of force are marked m, and the 
others n. 

Q. 295 — Could not a machine be used with such a winding? 




I II 

-o 



U-O 



^^ 



Power, N.r. 



IV III 



FIG. 168. 



A. — Yes; many such are in use. But the two sets of coils are 
connected up separately, with separate pairs of collector rings, 
and usually supply two separate circuits, as shown by Fig. 168. 



122 



Phase Difference. 



Q. 296 — What is such a machine called? 
A. — A two-phase alternator. Two-phase means that the arma- 
ture generates two distinct E.M.Fs. differing in phase. 



m-=o 




Power, N.T. 



Q. 297 — What does differing in phase mean? 

A. — That the two E.M.Fs. rise and fall at different instants. 
Fig. 169 shows the E.M.F. curves of the two sets of armature 
coils. 




Power, N. Y. 



FIG. 170. 



Q. 298 — What do the figures indicate? 

A. — Fractions of a cycle. Thus, when the coils, m, have passed 
through 14 oi a. cycle, the coils, n, are just beginning one, and so 



Three-Phase Alternator 



123 



on, the n coils being always one quarter of a cycle behind the m 
coils. 

Q. 299 — ^Are more than two sets of coils ever used? 

A. — Yes; alternators are built to give three phases, in which 
case three sets of armature coils are used. 

Q. 300 — How are they arranged? 

A. — Generally as shown in Fig. 170. Each set occupies one- 
third of the available space around the circumference of the arma- 
ture, and the three sets reach their maximum E. M. F's at equal 
intervals, one-third of a cycle apart, as shown by the three curves, 
Fig. 171, which shows two cycles. In Fig. 170 the different sets of 
coils are distinguished by making one set white, one black and 

13 3 




FIG. 171. 



one shaded. In Fig. 171 the curves of the three sets are identified 
by the letters W (white), B (black) and 5 (shaded). 

Q. 301 — Has each set a separate pair of collector rings, like a 
two-phase armature ? 

A. — No. There are only three collector rings usually; four can 
be used, but the fourth is not generally employed. Fig. 172 is a 
diagram of the armature connections; the letters, B, W and 5" in- 
dicate the black, white and shaded coils of Fig. 170. 

Q. 302 — Are the three rings arranged in a triangle, as drawn? 

A. — No; they are on the armature shaft side by side. 

Q- 303 — If ^11 three coils are connected in series, as in Fig. 172, 
why does not the current simply pass around through them ? 

A. — Because the E.M.F's of the coils are out of phase; i. e., 



124 Thrce-Phase Armature Connection. 

they rise to maximum at difterent instants, and the sum of the 
three at any instant equals zero. So, if the circuit wires were dis- 
connected no current could flow in the ^:hree armature windings. 

O. 304 — How can the sum of three E.M.Fs. be zero? 

A. — Because two of them always oppose the other one, except 
when one is at zero; then the other two oppose and neutralize each 
other. Reference to the curves of Fig. 171 will show that when- 
ever the E.M.F. of one coil is maximum in one direction the 
added E.M.F's of the other two are equal to it, but in the op- 
posite direction. For example, at i the E.M.F. of B is 1,400, say, 




FIG. 172. 

in the positive direction, while the E.M.F. of W and that of 5 are 
each 700 negative, or 1,400 negative combined. Thus^ they equal- 
ize the E.M.F. of B. At 2, the E.M.F. of B is 1,212 volts positive, 
while that of vS is 1,212 volts negative and W is zero — a condition 
of perfect balance. At j, the E.M.F. of 5" is at negative maximum 
while B and W are each 700, or one-half maximum, in the op- 
posite direction, maintaining the equilibrium of the three E.M.F's. 
This condition of equilibrium exists at every instant during a 
cycle. 



CHAPTER VIIL 

ALTERNATING CURRENT CIRCUITS -ALTERNA- 
TOR FIELD EXCITATION* 

Q. 305 — How are the circuits of three-phase machines ar- 
ranged ? 

A. — As in Fig. 173. The receiving devices are connected across 
each pair of wires. 

O. 306 — Is not Fig. 173 the same as the three-wire system of 
direct-current work ? 

A. — Not at all. The E.M.F. between I and II is that due to 
the coils W. Say this is 1,000 volts, effective. Then the E.M.F. 
from II to III supplied by the coils B is also 1,000 volts, and sim- 




II 



III 



s 



Power, N. Y. 



FIG. 173. 



ilarly the E.M.F. between I and III is 1,000 volts, because it is 
supplied by the coils S, and each set of coils gives the same 
E.M.F. 

Q. 307 — But why is not the E.M.F. of the coils B added to that 
of the coils W? 

A. — It is, but the E.M.Fs. are out of phase, as before described, 
to such an extent that the sum of the E.M.F's generated by two^ 
sets of coils never exceeds the maximum of one of them. Fig. 
174 is a diagram showing the relation of the three E.M.F's to the 
circuits, at the instants i, 2 and j, in Fig. 171. It is plain that the 
sum of, or the difference between, the E M F's from I to II, and 



126 



Three-Phase Circuits. 



from II to III, always equals and opposes the E.M.F. direct from 
I to III. 

Q. 308 — How are the connections arranged for four collector 
rings on a three-phase alternator ? 




T 

700 



T 1 



^JlTOO 



1400 



■^ ^III 




11 1212 s 



1212 




T 

700 



T— I 



i'. II 1400 



00 



Power, N.V, 



1 2 

fig: 174. 

A. — As in Fig. 175. Here the coils are not connected in a con- 
tinuous closed circuit, as in Figs. 172 and 173, but one end of each 
coil is connected to a common ring, and the other ends to indi- 
vidual rings. Four line wires are used, as shown. The E.M.F. 
between I and II is the sum of (or difference between) those of vS 
and W; between II and III, the combined E.M.F's of W and B, 
and bewteen III and I it is the combined E.M.F's of B and 5. 
Between IV and any other wire the E.M.F. is simply that due to 
the one set of coils in circuit there. 

Q- 309 — Does this arrangement balance like Figs. 172, 173 and 
174? 

A. — Yes; but there is a difference in line pressures. With the 
same alternator and other conditions, the effective E.M.F. be- 



l- 
li - 
III- 

IV- 




Pffieer.y.r. 

FIG. 175. 



tween any two of the three principal wires, I, II, III, is 1.732 
times the E.M.F. in the arrangement shown by Figs. 172 to 174, 
because two sets of coils are in series between each pair of wires. 
Q. 310 — What is the fourth wire for ? 



Three-Phase Circuits. 



127 



A. — To secure greater independence between the three legs of 
the system. Fig. 176 shows the elementary plan of this type of 
distribution. In practice, however, the fourth wire and collector 



IV 




II 



oimw 



III 




3-PHASE ryiOTOR 

FIG. 176. 



mnn 



{IHH I 



IHTH 



Power, N. T. 



ring are not often used, and the circuit connections are as in Fig. 

177. 

Q. 311 — Are there any distinguishing names for the two ar- 
rangements of three-phase armature windings ? 

A. — Yes. The armature arrangement in Figs. 172, 173 and 174 
is called the "delta" connection, and that in Figs. 175 and 177 is 
the "star" or "Y" connection. 

Q. 312 — Do the E.M.F's of a two-phase system balance like 
those just described? 



iiU I I 



m — f- 



6 6 6 6 




FIG. 177. 



A. — No; it is not necessary, because the two circuits are usually 
kept entirely distinct, as in Fig. 168. Sometimes the two middle 
wires are consolidated into one, as in Fig. 178. Even then a zero 



128. Two-Phase Circuit. 

balance between the E.M.F's is not essential, nor could it be ob- 
tained with the ordinary arrangement of armature windings. 

Q. 313 — Is not this like the three- wire direct-current system? 

A. — Precisely, except as to the amount of saving and ease of 
regulation. The windings m and n may be considered as separate 
armatures working in one fields instead of in separate fields as in 
the continuous-current system. But, as the E.M.F's are out of 
phase, the total E.M.F. from I to III is not twice that from I to II, 
but 1. 414 times that value. Thus, if the E.M.F's from I to II and 
from II to III are each 100 volts, the E.M.F. measured direct from 

I to III will be 141. 4 volts. This would not maifitain the load if 

II were removed; hence, even with a perfectly divided load there 
is current in II. In fact, the current in II will be 1.414 times that 
in I or III if the system is perfectly balanced, because it is the 





• — <i — i — u — (» — •►-11 



Power, N. 1*. 

FIG. 178. 



combination of two equal currents a quarter of a cycle apart in 
phase. Therefore, the middle wire must be about i^ times the 
area of each outer wire. 

Q. 314 — Then alternating currents differing in phase combine 
just as the E.M.F's do? 

A. — Exactly. Two equal E.M.F's a quarter-cycle (commonly 
designated 90 degrees) apart as to phase, if combined in series 
give a resultant E.M.F. 1.414 times the value of each original 
E.M.F. The same is true of equal currents a quarter-cycle apart 
in phase, combined in parallel. 

Q. 315 — What is the result when the difference of phase is 
something else than a quarter-cycle ? 

A. — The greater the difference in phase the less will be the 
resultant E.M.F. Resultant E.M.F's or currents can be plotted 
by the parallelogram of forces if we resort to the fiction of angular 
displacement to represent the difference in phase. For example,. 



Combining E.M.Fs. Out of Phase. 



129 



-suppose the E.M.F. or current to be supplied by a bipolar gen- 
erator (which is never done). Then draw the circle through 
which any given armature wire passes in one revolution, and let 
the radius of the circle represent the maximum E.M.F. (or maxi- 




mum current) generated by those armature coils which are con- 
nected in series. Thus, if the maximum E.M.F. is 1,000 volts, 
and we let each -J inch of radius represent 100 volts, the circle 
will have a radius of I5 inches. Now draw^ a horizontal line 
through the center, as in Fig. 179, and call that the zero line. 
Then a perpendicular line from this to any point on the circum- 




ference of the circle will represent the E. ^I. F. when the central 
wire of the armature coil is at a part of its travel corresponding 
with this point. Compare Figs. 179 and 180. The E.M.F. gen- 
erated by the winding when its center is in each position shown 



130 Combining E.M.Fs. Out of Phase. 

by the dot, Fig. 180, is indicated by the corresponding perpen- 
dicular Hnes in Fig. 179. This demonstration can be carried 
around the whole circle, of course. Here a cycle corresponds 
literally with a circle; a quarter-cycle is, therefore, 90 degrees of 
a circle; a sixth of a cycle is 60 degrees of a circle, and so on. And 
a cycle is always considered as equal to a circle, and parts of a 
cycle are represented by degrees of a circle, no matter how many 
magnet poles there are. 

Q. 316 — Then a difference of phase of an eighth of a cycle 
would always be equal to 45 degrees of a circle? 

A. — Exactly. It is not necessary to draw the circle, only the 
radius lines being required to compare E.M.F's differing in phase. 




Power, N. T. 

FIG. 181. 

For example, if we have two E.M.F's of 100 volts each, differing 
in phase by -J cycle or 45 degrees, we simply draw two lines, a and 
d, Fig. 181, radiating 45 degrees apart from a common center. 
The length of each line represents the E.M.F.; thus, the lines are 
i^ inches long, which means that each -J of an inch represents 10 
volts. Now construct the parallelogram exactly as in mechanical 
work and draw the diagonal, C. Then every eighth of an inch 
of C will mean 10 volts resultant E.M.F. In the diagram C is 
2 ^ inches long, which, at the scale of a and d {10 volts per -J 
inch), means a resultant E.M.F. of 185 volts. 

Q. 317 — Suppose the E.M.F's are not equal? 

A. — Then make the lengths of a and d differ as the E.M.F's 



Combining E.M.Fs. Out of Phase. 131 

differ in value; construct the parallelogram, and its diagonal will 
give the value of the resultant E.M.F. 

Q. 318 — Does this refer to effective or maximum E.M.F. ? 

A. — Either. If a and d are drawn to scale with the effective 
values, which is customary, then C gives the resulting effective 
value. If you start with the maximum values the resultant will 
be the maximum, and must be divided by 1.414 or multiplied by 
0.7071 to reduce it to the effective E.M.F. or current, as the case 
may be. 

Q. 319 — Is the current spoken of as effective and maximum 
also ? 

A. — Yes. It fluctuates exactly like the E.M.F. does, of course, 
and the current values in amperes are treated exactly like the 
E.M.F. values in volts. 

Q. 320 — Cannot the combined E.M.F's. be found without 
drawing diagrams? 

A. — Yes. If two E.M.F's of different phase, but equal values, 
be combined the resultant E.M.F. will be B^ = {E^ ^ B^) X 
cos (i @) where B^ and £^ are the two E.M.F's and cos (^ 0) is 
the cosine of one-half the angle of difference in phase. For ex- 
ample : We had (Q. 316) two E.AI.Fs. of 100 volts, 45 degrees 
Q cycle) apart. Applying the above formula: E^+ E^ is 200; of 
% is 22^ degrees, hence the cosine^ of (i 0) is .92388. Then we 
have 200 X -92388 = 184776, while the diagram gave 185 
as the resultant E.M.F. E^ . The formula is more accurate, 
of course, as fine diagram measurements are impracticable, even 
when one is expert enough to draw the diagram with accuracy. 
When the angle is greater than 90 degrees, however, its comple- 
ment must be taken. 

Q. 321 — Does this formula also apply to E.M.F's of different 
strengths and different phases? 

A. — No; the formula for such cases is 



Er = l^ {E,; + E^') + (2 X ^a X^b >^COS.©) 

These formulae are tedious to apply, particularly the latter one. 
Table IV will enable one to ascertain the resultant of the two 
E.M.F's, or two currents differing in phase, by making a few 
simple computations as explained at the foot of the table. 



* Ascertained from a table of sines and cosines. 



132 



Alternator Field Excitation. 



Q. 322 — Is an alternator field magnet excited like that of a 
direct-current dynamo? 

A. — Yes and no. It is excited in the same way — by passing 
direct current through the coils ; but this current, of course, 
continuous current through the coils ; but this current, of course, 
cannot be taken from the alternator brushes. It is supplied by 
an entirely separate direct-current dynamo, called the "exciter" — 
usually a 125-volt dynamo. 

TABLE IV. 

For ascertaining the resultant of two E.M.Fs. in 

series or two currents in parallel. The smaller 

of the two, X the figure in the body of 

the table = the resultant 





Difference of phase between the two 




* 


E.M.Ks. 


or currents 


in circular 


degrees. 


^ 


.2 


30° or 


45° or 


60° or 


90° 


6 


(4 


150° 


135° 


120° 


Pi 


1§ 


1.9319 


1.8478 


1.7321 


1.4142 


1§ 


1.05 


1.9802 


1.8940 


1.7755 


1.45 


1.05 


1.1 


2.0286 


1.9406 


1.8193 


1.4866 


1.1 


1.15 


3.0771 


1.9872 


1.8635 


1.5241 


1.15 


1.2 


2.1257 


2.0339 


1.9079 


1.5631 


1.2 


1.25 


2.1743 


2.0809 


1.9526 


1.6008 


1.35 


1.3 


2.2229 


2.1281 


1.9975 


16401 


1.3 


1.35 


2.2717 


2.1753 


2M27 


1.68 


1.35 


1.4 


2.3205 


2.2226 


2.0881 


1.7205 


1.4 


1.45 


2.3694 


2.2700 


2.1337 


1.7614 


1.45 


1.5 


2.4183 


2.3176 


2 1794 


1.8028 


1.5 


1.55 


2.4672 


2.3653 


2.2254 


1.8446 


1.55 


16 


2.5162 


2.4130 


3.2716 


1.8868 


1.6 


1.65 


2.5652 


2.4609 


2.3179 


1.9294 


1.65 


1.7 


2.6143 


2.5088 


2.3643 


1.9723 


1.7 


1.75 


2.6634 


2.5568 


2.4109 


3.0156 


1.75 


1.8 


2.7125 


2.6049 


2.4576 


3.0591 


1.8 


1.85 


2.7617 


2.6531 


2.5045 


2.1029 


1.85 


1.9 


2.8108 


2.7013 


2.5515 


2.1471 


1.9 


1.95 


3.8601 


27496 


2.5985 


, 2.1915 


1.95 


2. 


2.9093 


2 7979 


2.6458 


3.2361 


2. 


2.1 


3.0079 


2.8948 


2.7404 


2.3259 


2.1 


2.2 


3.1065 


2.9919 


2.8355 


2.4166 


2.2 


2.3 


3.2053 


3.0891 


2.9309 


3.508 


2.3 


2.4 


3.3041 


3.1865 


3.0265 


3.6 


2.4 


2.5 


3.4029 


3 2841 


3.1225 


2.6926 


2.5 



* The larger E.M.F. or current h- the smaller, thus 

E C 

— or, -• 
e c 

§ Equal E.M.F.''. or currents. 

Q. 323 — Why cannot a commutator be put on to give con- 
tinuous current for the field? 

A. — Because of the vicious sparking that would occur. Such 
a commutator is used occasionally to pass the main current 
through series field coils, in order to secure the same efifect as 
that of the compound field winding on a direct-current dynamo. 



The Rectifying Commutator. 



133 



The connections are as in Fig. 182, from which it will be seen 
that the commutator here, c, is arranged differently from the 
commutator of a direct-current dynamo. To distinguish between 
them this device is called a rectifier. The rectifier is shown off 
to one side in the cut for the sake of clearness. In the actual 
machine it is mounted on the shaft beside the collector rings. 




FIG. 182. 

Q. 324 — What are the wires inside the commutator for? 

A. — To show that alternate segments are connected together. 
In practice, wires are not used; the commutator, or rectifier, is 
built in two parts like a dental clutch coupling, with insulation 
between the two halves. Fig. 183 shows this construction. The 
total number of teeth or segments must equal the number of poles 
on the field magnet, so that the connections between the armature 
and field windings will be reversed every time a coil passes from 
pole to pole. 




FIG. 183. 



Q. 325 — In Fig. 182 only one set of field coils is shown. How 
is the exciter connected? 

A. — Fig. 184 is a diagram of all of the field connections where 



^34 



Lag of Current. 



series coils and a rectifier are used. In polyphase (two or three- 
phase) machines, and in a great many simple alternators, recti- 
fiers are not used. 

Q. 326 — Why would a rectifier spark worse if all of the field 
coils were supplied through it? 

A. — Because the brushes would then short-circuit the whole 
armature every time they touched two segments, as shown by 
Fig. 185, which shows the way the connections would be made. 
As the brushes pass from segment to segment the field coils and 
armature winding are both short-circuited. In Fig. 184 only the 
field coils can be short-circuited. 

Q. 327 — But does not this occur when the armature coils are 
in neutral position? 

A. — Yes; so far as the generation of E.M.F. is concerned. But 



m^osmmmmm' 



\TDR FIELD COILS 




Power, N. T. 



FIG. 184. 



even when the armature coils are cutting no lines of force there 
is quite an appreciable current flowing through them under usual 
conditions. 

Q. 328 — How can current exist when the E.M.F. is zero, as it 
is between each reversal? 

A. — Because the current strength doe^; not usually rise and fall 
precisely with the E.M.F., but ''lags" behind it a greater or lesser 
amount, according to circumstances. Fig. 186 shows the way the 
rise and fall of the current *'lags" behmd that of the E.M.F.* 
Now, if the brushes short-circuit the armature at the instant A, 



* The amount of difference in phase between current and E.M.F. is called the "angle 
of lag," and measured in degrees of a circle, exactly like the difference phase of two 
E.M.F.'s or currents, a complete cycle being represented by a circle, as previously 
explained. 



Commutating a Lagging Current. 



135 



when the generated E.M.F. is zero, the current will not be zero, 
but will have considerable strength, as represented by the dis- 
tance from the zero line up to the current curve along the line A. 
Then when the rectifier segment passes beyond the brush the 
path through which the current has been flowing from segment 
to segment will be broken and a serious flash will result. 

Q. 329 — What happens when the brush touches both segments 
at the instant the current is zero? 

A. — That instant is represented by the vertical Hne F, in Fig. 
186, which shows that the E.M.F. generated then would be con- 




FIELD COILS 

FIG. 185. 

siderable; hence, though the current in the outside circuit would 
be zero, the E.M.F. short-circuited in the armature would set up 
a separate heavy current through the armature wires, which 
would be interrupted when the brush left the segment, causing 
a destructive flash. The diagram shows a current "lag" of about 
^V cycle, or 18 degrees, which is frequently present in practice. 
At this amount of lag the E.M.F. has ro of its maximum value 
when the current is zero; similarly, the current strength is ts 
of its maximum when the E.M.F. is zero. For example, if a 
2,000-volt alternator were supplying 50 amperes, with the lag 
shown, the instantaneous value of the E.M.F. at V would be 



136 



Commtitating a Lagging Current. 



848J volts ( TO of 2,828.4 volts; see Q. 217). Short-circuiting an 
armature under such an E.M.F. would set up a tremendous cur- 
rent in the windings. If, on the other hand, we "commutate"t 
at A, when the armature is zero, the armature current will be 
25 1 amperes ; and although it is better to short-circuit this 
current than to short-circuit the 848-I volts of E.M.F., the flash- 
ing at the brushes would be pretty vicious. 

A 




Power, NTT. 



FIG. 186. 



Q. 330 — Why cannot the current be made to rise and fall in 
phase with the E.M.F. ? 

A. — Because it is retarded by self-induction, which is present 
in nearly every alternating-current circuit. 



tWhen the brush passes from one segment to the other the current is "commutated." 



CHAPTER IX. 
ALTERNATING CURRENT PRINCIPLES* 

Q. 331 — What is self-induction? 

A — The magneto-electric inductive action of an electro-mag- 
met. Therefore, any circuit including electro-magnets will show 
more or less self-induction when an alternating curent passes 
^through it. 

Q. 332 — What causes the inductive action? 

A. — The cutting of its own lines of force by the wires of the 



\ 






Waiver, N.Y- 
FIG. 187. 

■magnet coil. This generates an E.M.F. in opposition to the 
E.M.F. of the dynamo and ''chokes" back, or retards, the cur- 
rent flow. 

Q- 333 — How can the lines of force passing from pole to pole 
'Of a magnet be cut by the wires wound on the core of the magnet ? 

A. — Figs. 187 to 190 will show. Passing a current through a 

wire creates lines of force which form closed loops or circles 

around the wire, as indicated by Fig. 187. These magnetic loops 

•or circles do not suddenly spring into existence in their position 

■of maximum strength, but develop from a central point (the 



^38 



Self -Induction. 



center of the wire), expanding into larger and larger circles as 
the exciting current increases in strength, until it reaches maxi- 
mum. When the current begins to decrease in strength the 
magnetic circles collapse successively, each finally vanishing at 
the central point when the exciting current is zero. The wave 
circles formed by dropping a stone in a pool of water illustrate 
the expanding action of the magnetic lines. Their contraction, 
or collapse, is simply the reverse of this. Now, if several wires,, 
all carrying current, be put together the magnetic loops created 
by them will unite into one set of loops circling around the gen- 
eral center of the group, as in Fig. i88, where the dot represents 
the ''center of propagation." It is clear that when the lines of 
force expand outward from the central point they are cut by all 




////////;;^^>.\\\\V\\\\^ 





FIG. 188. 



Power, m.Y. 

FIG. 189. 



of the wires of the group, and when they collapse they are cut by^ 
the wires in the opposite direction. As previously explained,. 
E.M.F. is induced by cutting lines of force with a conductor, ir- 
respective of whether the lines are stationary and the conductor 
moves, or the conductor is stationary and the lines move (side- 
wise). In this case the conductor is fixed and the lines move 
past it and back again, inducing an E.M.F. opposite in direction-, 
to the E.M.F. which drives the current through the wires, but not 
so great, of course, in value. 

Q. 334 — But what has this to do with an electro-magnet? 

A. — The magnetic lines circle around the wires of the magnet 
coil just as they would around a group of straight wires. For 
example, if the single wire of Fig. 187 were bent into a ring the 
magnetism would circle around it, as in Fig. 189, just the same 



Self-induction. 



139 



And the magnetic loops about any section of a coil of wire are the 
same as though the wires were straight, as shown in Fig. 190. 
At the instant of time represented in the cut the lines are flowing 
from top to bottom in the core. Now, when the current in the 
coil falls to zero the lines of force collapse, cutting across the 
wires of the coil and inducing an E.M.F. in it. The action takes 
place twice in every cycle, and is called ''self-induction," because 
the lines of force created by the coil induce an E.M.F. in it. 

Q. 335 — How much E.M.F. is induced in the coil? 

A. — That depends upon the number of turns of wire, the num- 
ber of lines of force, and the frequency (number of cycles per sec- 
ond). The value of this E.M.F., called the inductive or reactive 

/ ,- ^--/^<\\\\\\W'/////'/.> — ^. ^N \ 

/ / /' ___ '''•Cs \\\\\\'.?//////^ C-'^ N N ^ 

' / / / ' / /''QS""v'>''*\'i"',r'';''','/'///' QQ "~\ ^ \ \ \ \ \ 
' ' ' //^v^'w iilX |''|^''^^^^ ' 1 ' 1 1 I 1 

f I M M \v^Sii'iiSK^v/ /^ ^ I t t t 






Power, N. T. 



FIG. 190. 



E.M.F., is found by a formula similar to that for continuous- 
current E.M.F., namely, 

i.iiX^XiVx/ 4>XA^X/ , .. 

E-, r= or •^— (28) 

25,000,000 22,522,522.5 

In this formula <l> represents the maximum number of magnetic 

lines created; N is the number of complete turns of wire; and f 

is the number of cycles per second. The inductive E.M.F. may 

be also calculated as below: 

^i=27rXCX/Xl^ (29) 

Here L represents the "inductance" of the coil. 
Q. 336 — What is meant by the inductance? 
A. — The coefificient of self-induction. In formula shape, 

^ = A^ V ^ (30) 

141,421,360 X c 
Here <I> and TV retain the significance given under Q. 335, and C 



140 Reactive E.M.F. 

is current, of course. The relation between formula (28) and 
formula (29) may be seen by including formula (30) in formula 
(29), thus: 

^=6.2832 X cx/x ^ . ^ ^ 

141,421,360 X c 

which reduces to formula (28), thus : 

^ ^ ^ X NX/ 

' ~ 22,522,522.5225 

Q. 337 — Does not the inductive E.M.F. in opposition to the 
dynamo E.M.F. offset part of it? 

A. — Yes. It neutralizes part of the impressed E.M.F., and 
the result is that the amount of current that flows through the 
coil is much smaller than it would be if the current were not al- 
ternating. The inductive E.M.F., however, is not fully opposed 
to the impressed E.M.F.,"^ so that it does not neutralize its full 
value in the latter. 

Q. 338 — Why is it not fully opposed to the impressed E.M.F.? 

A. — Because it always lags 90 degrees behind the current, and 
the current lags behind the E.M.F. less than 90 degrees. If the 
current lagged 90 degrees the inductive E.M.F. would be 180 
degrees, or -J cycle, behind the impressed E.M.F., and thus op- 
pose it squarely. As it iS; its opposition is exactly similar to the 
opposition of one mechanical force against another at an angle, 
so that its full force is not presented against the impressed force. 

Q. 339 — How may the amount of opposition be computed? 

A. — The amount of direct opposition is not usually computed. 
The value of what remains available of the impressed E.M.F. is 
the essential one, because it is that that forces the current 
through. It may be found as follows: 

Subtract the square of the inductive E.M.F. from the square of 
the impressed E.M.F. and take the square root of the remainder. 
This root will he the E.M.F. actually available for work. 

Thus: 

I/E' — E\ = E. 

This available E.M.F., Ea, is called "active" E.M.F., because it 
alone forces current through the coil. If for any reason one de- 



* Impressed E.M.F. is the E.M.F. supplied by some outside source, as distinguished 
from the inductive or back E.M.F. of the coiL 



Reactive E.M.F. — Impedance. 141 

sired to know the effectual oposition of the inductive E.M.F., 
simply subtracting E a from E will give it. It must be remem- 
bered, however, that the effectual opposition thus obtained is not 
the full value of E^. 

Q. 340 — Can the active E.M.F. be ascertained without know- 
ing the inductive E.M.F.? 

A. — Easily, if the resistance of the coil or circuit is known. 
Then C y^ R ^ Ea. And the inductive E.M.F. can also be found 
by the formula 

E. = v^^^^^ToTRn 

obviating the calculations given under Q. 635 and 636. 

Q. 341 — Is not the formula C X R =^ E^ like Ohm's law? 

A. — Exactly. The current X the resistance equals the E.M.F. 
that actually forces it through. Or^ the current is equal to the 
E.M.F. actually available to force it through a resistance, divided 
by that resistance. Thus, 

R -^' 

Q. 342 — Then the product of C X -^ does not equal the dy- 
namo E.M.F., like it does in continuous current work? 

A. — No, because part of the dynamo current is neutralized, 
and therefore not available. Looking at it from another direc- 
tion, however, we can apply the principle of Ohm's law to the 
impressed E.M.F. by taking into consideration all of the oppo- 
sition encountered by the impressed E.M.F., instead of merely 
the resistance. This resistance, R, of the magnet coil opposes 
the current flow, and the self-induction also opposes it, but with- 
out loss of energy. Now, these two effects — resistance and ''re- 
actance" (called so because the self-induced E.M.F. reacts on 
the impressed E.M.F.) — added together at any instant, give the 
total opposition at that instant to the current flow. This total 
opposition is called ''impedance," and is represented by Z. 

Impedance is equal to the square root of the sum of the squares 
of the resistance and reactance. Thus, 

V Reactance" -f- Resistance" = Impedance; or, 

■VlC^R''— Z. 
As Z represents the total opposition to the current flow, we can 
write 



142 Reactance. 

-^= C, and — = Z, and C y, Z =1 E; 

which is plainly in accord with the principle of Ohm's law. 

Q. 343 — In what units are reactance and impedance measured? 

A. — In ohms, like resistance. R is the resistance in ohms due 
to the material of the conductor; X is the resistance in ohms due 
to self-induction, and is called reactance chiefly to distinguish it 
from the other resistance, and incidentally because it conveys an 
idea of the character of the inductive opposition. Z is the com- 
bined effect of these two, in ohms. 

Q. 344 — How can the reactance and impedance of a coil or 
circuit be ascertained ? 

A. — By simple measurements, if current is flowing. As £ -^ C 
= Z, the impedance may be found by dividing the E.M.F. by the 
current. And as i/r' ^ x" = Z, Z' = R' -{- X'; so that, hav- 
ing found Z, subtracting R' from its square gives the square of 
the reactance, or X^. Thus, if a coil has 10 ohms resistance and 
an alternating E.M.F. of 500 volts applied at its terminals forces 
only 20 amperes through, it is evident that there is reactance 
present, because if there were only resistance the current would 

be "-;^ = 50 amperes. £ -^ C = Z, so the impedance of this coil 

would be -^ =2^ ohms; and as the resistance is 10 ohms, the 
10 *^ 



reactance is V25' — IQ^ or V625 — 100, which is 22.9 ohms. 

Q. 345 — Suppose the resistance is not known? 

A. — Then the reactance is extremely difficult to obtain. Mathe- 
matically it is equal to the self-inductive E.M.F. divided by the 
current, or 

~= X; and ^ = C; and C X X = E,. 
C X 

But thevalue oiEi is not easy to ascertain in practical work unless 
the resistance is known (see Q. 340), as it depends upon the maxi- 
mum number of magnetic lines passing through the core of the 
magnet. 

O. 346 — What is the difference between self-inductive E.M.F. 
and reactance? 

A. — Reactance is the equivalent, in ohms, of the resistance 
through which the self-inductive E.M.F. would force the current. 



Impedance Diagrams. 



143 



Thus, if the current, C, were 20 amperes, and the resistance, R, 
were 40 ohms, and the reactance, X, were 35 ohms, the active 
E.M.F., £a would be C X ^ = 20 X 40 = 800 volts, and the in- 
ductive E.M.F., Ei would be C X ^ = 20 X 35 == 700 volts. 
According to the answer to Q. 339, the impressed E.]\I.F., E, is 

€qual to ^JE " + E'^. This would make it 1,000 volts in this case. 
And as £ -^ C = Z, the impedance must be 1,000 -^ 20 = 50 
ohms. Checking- this by the formula (33), Z — V-^'' + ^, we 
have V40^ + 30' = 50j which agrees perfectly. 

Q. 347 — Why do the values of E, £a, ^,, X and R have to 
be squared in. calculating their relations? 

A. — Because their relations are exactly analogous to those of 
mechanical forces combined at an angle and their resultant force. 
If we have two forces at right angles, as a, i, Fig. 191, acting upon 
a common point, the combined influence at the point will be in 




Power, N. Y. 




FIG. 191. 



Power, A. V. 

FIG. 192. 



the direction of r, according to the parallelogram of forces. And 
if the length of each line, a and i, be proportional to the force it 
represents, r will be proportional to the resultant force. Now, if 
a represents the active E.M.F. and i the E.M.F. of self-induction, 
then r will represent the total or impressed E.M.F. 

Q. 348 — How does this account for the squares? 

A. — A study of the transposed lines in Fig. 192 will show. It 
makes no difference whether a force be imagined as pulling upon 
a point or pushing against it; the result is the same if the effort 
is in the same direction. Fig. 192 shows the force i pulling in- 
stead of pushing, so that the two forces and their resultant form 
a right-angle triangle. And ''the square of the hypothenuse (r) 
of a right-angle triangle is equal to the sum of the squares of the 
other two sides." Hence, if a represents E i represents E 
and r represents E, then 

£' =£' + £' and E = y" ~E'~^^~^~ 



144 



Phase Relations. 



Q. 349 — Are E^ and Ei always at right angles with each 
other ? 

A. — Yes; invariably. 

Q. 350 — How about Z and X and R? 

A. — Resistances may be considered exactly like forces. In 
mechanics they are considered so. And as the dead resistance 
of R is opposed to E ^ and the inductive resistance X to E. these 
two, R and X, may be considered as active forces at right angles^ 
like a and i, Figs. 191 and 192. Their resultant resistance (im- 
pedance, Z) is as r. Hence, Z' = 7?== + X\ or Z — ^R' -f X". 
Fig. 193 shows these relations. For practical purposes it is more 
intelligible to simply consider that the inductive E.M.F. neutral- 
izes part of the impressed E.M.F.^ leaving the active E.M.F". 
available for work. 




ACTIVE E.M.F. Power, N.T. 

FIG. 193. 



Q. 351 — Is the E.M.F. neutralized by the inductive E.M.F. 
wasted? 

A. — No; it is rendered useless, but no energy is wasted thereby. 
The result of the neutralizing impressed volts of E.M.F. with in- 
ductive volts of E.M.F. is the same as though neither existed, so 
far as the work done is concerned. A mechanical analogue would 
be afforded by two locomotives of equal power, head to head, 
each trying to push the other backward. Each might be exert- 
ing a pressure of many tons against the other, but no work would 
be done (assuming, of course, that the wheels could not slip on 
the rails). 

Q. 352 — As the inductive E.M.F. neutralizes part of the im- 
pressed E.M.F., how can one ascertain the number of watts sup- 
plied to a magnet coil ? 

A. — The watts in any circuit are the product of the current and 
the E.M.F. that forces it through. In alternating current work 



Apparent Watts — Angle of Lag. 145 

this E.M.F. is the active E.M.F., Ea previously explained. 
Therefore, £a, X C =: j^. 

Q. 353 — What is obtained by multiplying the impressed 
E.M.F. by the current? 

A. — The ''apparent" watts, which may be represented by W? ; 
so-called because they represent the power which seems to be 
applied. The actual power is called "true" watts, or "real" watts, 
which are the product of E^ and C, as above. If one knows how 
much the current lags (see answer to Q. 328) behind the E.M.F. 
(impressed), the real watts may be calculated by the formula: 
Imp. E.M.F. X Current X cosine of 6 = Watts; or Appar. Watts X cosine 

ofe. 

Ex C X cos e = W; or IVf X cos = JV. 

Here the symbol © represents the number of degrees of a 
circle by which the current hangs back in its rising and falling, 
as compared with the rising and falling of the E.M.F., i. e., the 
"angle of lag." 

Q. 354 — How may the angle of lag be ascertained? 

A. — If the resistance of the circuit or coil be known, the cosine 
of the angle of lag may be easily found by the formula 

Cx J? 

— -^ — = cos 9. 

From a table of sines and cosines the value of @ can be readily 
found. 

Q. 355 — Can the angle of lag be measured? 

A. — Not directly, but its cosine can be obtained by measuring 
the true watts with a wattmeter (not a watt-hour meter) and the 
apparent watts with a voltmeter and ammeter, and dividing the 
one by the other. For example, if the wattmeter indicates 1,600, 
the voltmeter 100 volts and the ammeter 20 amperes, the true 
watts will be 1,600 and the apparent watts 100 X 20 ^ 2,000. 
The ratio between the two is UU = 0.8, which will be the cosine 
of the number of degrees by which the current hangs back be- 
hind the E.M.F. 

Q. 356 — Why does not the wattmeter indication agree with the 
product of the voltmeter and ammeter indications ? 

A. — Because it will only indicate the true or real watts, and the 
product of the £ X C gives the apparent watts. 



146 Power Factor. 

Q. 357 — Why does dividing the real watts by the apparent 
watts give the cosine of the angle of lag ? 

A. — Because the real watts =3 £ x C X cos @ and the appar- 
-ent watts 

^xCxcose Eye 

— £ y^ (^ — = 'e'^^c X cos e 1^ I X cos e = cos e 

Cos O is equivalent to what is known as the. "power factor" of a 
circuit or apparatus in which the current lags % degrees behind 
the E.M.F. 

Q. 358 — What is meant by power factor? 

A — The ratio between the true watts and the apparent watts, 
or the proportion of the apparent watts that is available for power. 
In formulas it is represented by either cos or the letter p. 



Mathematical Relations. 



U7 



Impress. E. M. F. 
Imp. E. M. F. 
Imp. E. M. E. 
Imp. E. M. F. 

Inductive E. M. F. 
do 
do 
do 



Active E. M. F. 
do 

do 
do 

Current 
do 
do 
do 
do 

Impedance 

do 

do 
do 

Resistance 
do 

do 
do 

Reactance 
do 
do 
do 

Inductance 

App. watts 
do 

True Watts 

do 

do 
do 

Power factor 

do 
Sine of Lag Angle 

do 



v/jnduc. E.M.F.'^+ Act.E.M.F.^, or i^ =. t^£:' -j- £^' 
Indue. E.M.F.-^sine of lag angle, ox E = E^^ sin 6 
Act. E.M.F.-^ cosine of lag angle, or E = E^-^ cos B 
Current X Impedance, or E = C x Z. 

v/I^^p"^E7A07"^^^AcrEyM7FA or ^, = l^E' - E^ 
Imp.E.M.F. X sine of lag angle, ox E^ = E Y. sin 9 
Current X Reaciance, or ^^ = C X X. 

2 TT X Frequency X Inductance 

X Current, or^i = 2vrX/X^XC 

\/lmp. E.M.F.-'— indue. E.M.F.^ or E^ = v^ E' — E'^ 
Imp. E. M. F. X cosine of lag 

angle, or E^ = Ex cos Q = EX p 

Current X Resistance, or E„ = C X ^. 

True watts -=- Current, or iS'^ = IV ^ C. 

True watts -4- Active E. M. F.. or C = W ^ E^ 
\/True watts -^ Resistance, or C = i^ W -^ R. 

Imp. E. M. F. -^ Impedance, or C = ^ ^ Z. 
Indue. E. M. F. ^ Reactance, or C = E^ ■*■ X. 
Active E. M. F. -^ Resistance, or C = E^-^ R. 

■■ -v^ Resistance^ 4- Reactance^, or Z = f^R^ + ^^ 

R 

: Resistance -^ cosine of lag angle, or Z = R -^ cos 6 or — 

p 

: Reactance ^ sine of lag angle, or Z = JT -*- sin O 

: Imp. E. M. F. — Current, or Z = E ^ C. 

: s/lmpedance^ — Reactance^ or R = */ Z'^ X"^ 

: Impedance X cosine of lag 

angle, or 7? = Zx cos 9 or Z X / 

: Act. E. M. F. + Current, or R = E ^9. 

: Real watts -^ Current, or R = IV ^ C. 

-. *4mpedanee^ — Resistance^ or X = -s/z^ R^ 

■- Indue. E. M. F. -f Current, or X = E-^ C. 
: Impedance X sine of lag angle, or X= z'x sin 9 
: 2 TT X Frequency X Inductance, orX=2nxfXL. 

Magnetism X No. of wires 



or L = 



^ X N 



i,4<3.2i4X C- 
or W? = E X C. 
or ExC^Ox Z. 

or W ^ Ex C X cos 9 



~ 1,414,214 X current 

= Imp. E. M. F. X Current, 
= Current'^ X Impedance, 

= Imp. E. M. F. X Current X 

power factor, 
- Current X Impedance X power 

factor, or ^ = C^ x Z X cos 9 

= Active E. M. F. X Current, or W = E^X C 
= Current^ X Resistance, or W = C^ X R. 

= Resistance -*- Impendance, 

or;^ = Cos9= i^ + Z 
= Active E. M. F. •*■ Impress. 

E. M. F., orp = Cos 9 = ^„ -^ E. 

= Indue. E. M. F. + Imp. E.M.F., 



= Reactance ■*■ Impedance, 



or Sin 9 = E^ 
or Sin 9 = X 



E. 
Z. 



CHAPTER X. 
TRANSFORMERS. 

Q. 359 — Why is alternating current used when direct current 
is so much simpler ? 

A. — Because of the ease with which a high E.M.F. can be gen- 
erated and changed to a lower or higher E.M.F. 

O. 360 — Of what advantage is this? 

A. — It permits the transmission of large power to a great dis- 
tance economically. The higher the E.M.F. the less the current 
for a given amount of power, consequently the smaller the line 
wire may be for a given loss, or the smaller the loss with a given 
size of wire. For example, 1,000 kilowatts at 2,000 volts demand 
a current of only 500 amperes; if the E.M.F. were 500 volts the 
current would be 2,000 amperes. Xow, suppose we had a line 
that transmitted 500 amperes with a drop of 100 volts, or 5 per 
cent. The drop with 2,000 amperes would be 400 volts, or 80 per 
cent of the available E.M.F. of 500 volts. Hence, to transmit the 
1,000 kilowatts at 500 amperes with a loss of 5 per cent, the line 
would have to be sixteen times as heavy as it would with an 
E.M.F. of 2,000 volts. 

Q. 361 — Cannot direct current be generated at high potentials? 

A. — Yes, up to certain limits. Above 2,000 or 3,000 volts, how- 
ever, the commutator becomes prohibitively expensive, and it is 
extremely difficult to obtain smooth commutation at such high 
potentials. Moreover, the potential is too high to use at the 
lamps or motors, and to reduce it requires the use of a combinefl 
motor and dynamo, the motor winding being designed for the 
high potential and the dynamo winding for the lower distribution 
voltage. 

Q. 362 — How is alternating current E.M.F. lowered? 

A. — By means of an apparatus without moving parts, called a 
"transformer." 



Transformer Construction. 



149 



Q^ 25^ — What is a transfcrmer like? 

A.— It consists of an iron core of thin sheets, on which are 
wound two sets of coils called the primary and secondary wind- 




Power. N.V. 



FIG. 194. 



ings. Fig. 194 shows one form of transformer, with its case re- 
moved, and Fig. 195 shows it complete. 

Q^ 264 — How^ does a transformer act? 

A.— The primary winding, supplied by the high potential 
mains, creates Hues of force in the core, which may be considered 
to continuously and rapidly expand and contract; these lines are 




FIG. 195. 



''cut" by the secondary wires, and consequently an E.M.F. is in- 
duced in the secondary winding, which furnishes current to the 
bw-pressure mains. 



150 ■ Ratios of Tracts formation. 

Q. 365 — How does this change the E.M.F.? 

A. — The primary E.M.F. is not changed, but the magnetic Hnes 
created by the primary current induce a lower E.M.F. in the sec- 
ondary windings. 

Q. 366 — Why is the secondary E.M.F. lower than the pri- 
mary? 

A.=Because the secondary winding has fewer turns of wire 
than the primary, and the E.M.F. is proportional to the product 
of wires, flux and frequency. The frequency and flux are the 
same for both windings, the only difference being the number of 
wires. 

Q. 367 — Then the difference between the numbers of turns in 
the primary and secondary windings determines the difference in 
E.M.F.? 

A. — Yes. And for all practical purposes, the ratio of primary 
to secondary turns of wires may be considered exactly the same 
as the ratio of primary to secondary E.M.F's. That is, if the num- 
ber of turns in the secondary winding is one-tenth the number of 
turns in the primary winding, the secondary E.Al.F. will be one- 
tenth of the primary E.M.F. 

Q. 368 — How much lower is the secondary E.M.F. than the 
primary ? 

A. — There are several ratios. For ordinary commercial dis- 
tribution the primary E.M.F. is either 1,000 or 2,000 volts, and 
the secondary E.M.F. is either 50 or 100 volts, according to pref- 
erence.* Therefore, the ratio between primary and secondary 
may be 10 to i, 20 to i or 40 to i. The most common ratios are 
10 and 20 to I, the primary E.M.F. being 1,000 or 2,000 and the 
secondary 100 or the primary being either 1,040 or 2,080 and 
the secondary 104 volts. 

Q. 369 — How is the output of a transformer determined? 

A. — The E.M.F. may be approximately determined from the 
formula previously given for inductive E.M.F., viz.: 

. ^^ 4.44^X X NX / ^^j^ 

' 100,000,000 

in which ^ is the maximum magnetic flux in the core; A^, the 



* To be strictly accurate, some systems use 1,040 and 2,080 volts primafy E.M.F. and 
52 and 104 volts secondary E.M.F. Others use the even potentials above mentioned. 



Transformer Core Magnetization. 151 

number of turns in the coil, and /, the frequency (cycles per sec- 
ond) of the supply circuit. 

Q. 370 — Is this formula for the primary or secondary coil? 

A. — It applies to both. 

Q. 371 — How is the maximum magnetic flux, <l> determined? 

A. — It takes care of itself. In making the calculation for the 
primary coil, the value of £i may be considered as practically 
equal to that of the primary line potential. Then the flux may be 
approximated by the formula 

^ jgp X 100.000,000 / s^ 

- 4.44XA^pX/ • ^\' 

in which £p is the primary E.M.F., A^p the number of turns in 
the primary coil and f the frequency. The cross section of the 
core is usually made large enough to bring the magnetic density 
down to about 20,000 lines per square inch in transformers used 
on 133 cycle circuits. Several preliminary calculations are neces- 
sary before satisfactory proportions are obtained. 

Q. 372 — Does the secondary winding change the magnetic 
flux? 

A. — Not appreciably. The flux remains at practically the same 
value for all loads and no load in the secondary coil, as long as 
the primary E.M.F. is unchanged. 

Q- Z7?> — Does not the secondary current and winding tend to 
magnetize the core? 

A. — Yes, in the opposite direction to the primary; conse- 
quently, when current is allowed to flow in the secondary, the 
primary current increases just enough so that the combined effect 
of the two windings remains the same as the magnetizing effect 
of the primary alone when the secondary is open. 

Q. 374 — Are the currents the same in strength? 

A. — No; the ampere-turns are practically the same, though. 
Hence the ratio of primary current to secondary current is the 
same as the ratio of secondary turns (and E.M.F.) to primary 
turns (and E.M.F.). Thus, if the primary is wound for 1,000 volts 
and the secondary for loovolts the primary turns will be 100 times 
the secondary turns, and the secondary current will be 100 times 
the primary current at any and all loads. 

Q. 375 — What determines the amount of current a transformer 
can stand? 



152 



Size of Wire. 



A. — The size of the wire used in the winding, roughly. More 
accurately, the resistance of the winding and the efifective area of 
radiating surface. 

Q. 376 — What effect has the radiating surface upon the current 
capacity ? 

A. — The greater the effective radiating surface the less will be 
the rise in temperature for a given current in a given size of wire. 
Hence, the greater the radiating surface of a coil, the more cur- 
rent can be passed through it for a given rise in temperature. In 
ordinary practice the size of the wire forms a sufficiently good 
guide as to the amount of current allowable. 

Q. 377 — What area of wire is usually allowed per ampere of 
current ? 

A. — Fifteen hundred circular mils per ampere is a fair figure. 

Q. 378 — What rise of temperature is allowed? 



1000-voLT 

PRIMARY MAINS 






HHtf! 




FIG. 196. 



A. — There is no uniformity in the rise of temperature. Some 
transformers show a rise of 30 degrees and some a rise of 130 de- 
grees Fah. A good limit for average conditions is 80 to 90 de- 
grees Fah. 

Q. 379 — What regulates the amount of current furnished by a 
transformer ? 

A. — The resistance of the work circuit, just as in the case of a 
constant-potential dynamo. 

Q. 380 — Then the current increases as the resistance of the cir- 
cuit decreases ? 

A. — Yes. Overloading is prevented by fuses in both primary 
and secondary circuits. 

Q. 381— Where are the fuses located? 

A. — Within pockets either in or near the transformer case. 



Transformer Windings. 



153 



They are mounted on porcelain bases, similar to the ordinary fuse 
block, but of special shape to suit the different conditions. 

Q. 382 — How are transformers connected to the circuits? 

A. — The primary winding is connected to the two sides of the 
primary mains, and the secondary is similarly connected to the 
secondary work circuit, as in Fig. 196. The secondary terminals 
of a transformer may be considered as the terminals of a dynamo. 

Q. 383 — Are the transformer coils wound side by side, as in 
the diagram? 

A. — No; they are wound one over the other on the same part of 
the core. It is customary to draw them side by side merely to 
show that they are separate electrically. 

Q. 384 — Why has the transformer in Fig. 194 six terminals to 
its coils if there are only two windings ? 




s s 



A. — Because the secondary winding is divided into two distinct 
coils, which may be connected in series or in parallel, whereas the 
two primary coils are connected permanently in series. This ar- 
rangement is shown more clearly by the diagram, Fig. 197. 

Q. 385 — What is the result of changing the connections of the 
secondary coils ? 

A. — When connected in series they give twice as great an 
E.M.F. as when in parallel. 

Q. 386 — Does not this double the output of the transformer? 

A. — No. The output in watts remains unchanged because the 
current-carrying capacity is divided. Thus, if each coil can stand 
10 amperes and generates 50 volts, when connected in series the 
current capacity is 10 amperes and the E.M.F. 50 X 2 = 100 
volts. Connected in parallel, the current is 20 amperes and the 



154 



Transformer Connections. 



E.M.F. 50 volts. Hence, the watts are 1,000 in both cases. See 
Fig. 198. 

Q. 387 — Can transformers be worked together, like dyna- 
mos, at the secondary ends ? 



^ -100-votTs -*■ 

<. 50-VOtT-S *■ -^ 50-VOUT-S >- 



-TO-AMPERES- 



10 AMPERES 



20 AMPERES 



P»u>er,N.T. 




20 AMPERES 



A. — Yes; but it is not usually advisable to so work them be- 
cause several small transformers are less efificient than one large 
one, and it is seldom practicable to arrange for disconnecting one 
or more of a group as the load decreases, and putting them back 
as it increases. 

Q. 388 — When transformer secondaries are worked together, 
are they connected in series or in parallel? 

A. — They are connected in parallel for ordinary two-wire sec- 




Q 



Powei; N. r. 



FIG. 199. 



ondary circuits. The connections are exactly the same as though 
the secondaries were dynamo armatures. 

Q. 389 — Does it make any difference which way transformer 
terminals are connected to a line? 



Transformer Connections. 



155 



A. — They can be connected in either way, but they are almost 
invariably connected in parallel. 

Q. 390 — Can they be arranged to supply a three-wire system? 




Power, y. r. 



FIG. 200. 



A. — Yes. If they are worked together the same precautions 
must be taken as in the case of continuous-current dynamos — 




TX 



SWITCH AND 
CUT-OUT 



TRANSFORMER 



^ 



^ 



Power, y. r. 



FIG. 201. 



«. 



PLUG CUT-OUT 



'Mae 
00 



namely, for parallel grouping, like terminals, must be connected 
together, while for three-wire and series working, unlike terminals 
must be connected to each other. Fig. 199 is a diagram of two 



iS6 



Transformer Connections. 



transformers connected up for three-wire secondary service, and 
Fig. 200 is a diagram of a single transformer with its two sec- 
ondaries connected to a three-wire circuit. Fig. 201 is a semi- 
pictorial diagram of this latter arrangement. 

Q. 391 — Has alternating-current apparatus any positive and 
negative terminals ? 

A. — Not constantly, of course, because the polarity changes 
several thousand times a minute. The polarity at any given in- 
stant is what must be considered. For example, if two second- 
aries are to be connected to three-wire mains, as in Fig. 199, care 
must be observed to connect them so that when the current is 
flowing from a to h in A the current in B will be flowing from 
c to d. 

Q. 392 — How can one tell which way the current is flowing at 
any instant ? 

A. — This cannot be ascertained practically, but it is easy to find 




d 

Power, N.T. 

FIG. 202. 




6 6 



FIG. 203. 



Power, y. r. 



out whether the currents in the two secondaries are agreed or 
opposed by switching on one lamp on each side of the system 
and disconnecting the neutral wire at n, in Fig. 199. If the con- 
nections are properly made, the lamps will light; if not, they will 
not light. 

Q. 393 — What is the remedy if they don't light? 

A. — Simply reverse the connections of one secondary. 

Q. 394 — What precaution is necessary in connecting second- 
aries in parallel ? 

A. — Connect two ends with a piece of very small fuse wire (i 
ampere), as in Fig. 202. Then touch the remaining end, a, of one 
secondary to the free end, c, of the other. If the fuse does not 
blow, the connections may be made permanent, as in Fig. 203. 
If it does melt, then the connections must be reversed. 

Q. 395 — What would happen if the secondary winding were 
connected to a 50 or lOO-volt circuit and the primary winding to 
another work circuit? 



Step-Up and Siep-Dozvn Transformers. 



157 



A. — The secondary would then be the primary and induce a 
high potential in the other winding; if the primary were wound 
for 1,000 volts, that E.M.F. would be induced in it. 

Q. 396 — Are transformers ever used this way? 

A. — Often. Such a transformer is called a step-up transformer, 
because the voltage is raised. Step-up transformers are used to 
raise the generator E.M.F. for long-distance transmission in 
order to save wire. At the far end of the transmission line step- 
down transformers bring the line pressure down to a practical 
E.M.F. for distribution. See Fig. 204. Here the two trans- 
formers marked T are step-up and step-down transformers, and 
all those marked t are ordinary transformers. 



15.000 VOLT LINE 




FIG. 204. 



Q. 397 — AVhat is the difference between step-down transform- 
ers and the ordinary kind? 

A. — None, in principle. An ordinary transformer is a "step- 
down" one, but the term '"step-down" is applied only to those 
transformers which are built for high voltage at the secondary 
end and extremely high voltage at the primary end. 

Q, 398 — Why cannot the full-line potential be generated and 
furnished direct to the line by the generator ? 

A. — It can be, and is, done when the alternator is an inductor 
machine or has a stationary armature and revolving field magnet. 
When the more common form of machine (revolving armature) 
is used, it is inadvisable to generate very high potentials, because 
of the difficulty of maintaining the armature insulation. 



158 



Transformers on Polyphase Circuits. 



Q- 399 — Why is the insulation harder to maintain on a re- 
volving armature than on a stationary one ? 






a 



GENERATOR 

COLLECTOR 

RINGS 



-O- 





MOTOR 



FIG. 205. 



A. — Because, primarily, the wires cannot be so securely 
fastened to a moving core as to a stationary one; and, secondly, 




w 



GENERATOR 

COLLECTOR 

RINGS 




[wmm 



Power ^.r. 



FIG. 206. 



the centrifugal effect upon rapidly revolving coils tends to shift 
them and chafe the insulation. 

Q. 400 — Are transformers used on polyphase circuits? 



Transformers on Polyphase Circuits. 159 

A. — Yes, just as on any others. 

Q. 401 — How are they arranged? 

A. — They are connected to the primary wires as though each 
pair of those wires were a simple alternating-current feeder or 
main. Fig. 205 shows the connections for two-phase distribu- 
tion, and Fig. 206 shows typical connections for three-phase dis- 
tribution; the small circles in both diagrams represent lamps or 
other translating devices. 



CHAPTER XL 
ALTERNATING CURRENT MOTORS. 

Q. 402 — What is the advantage of polyphase distribution? 

A. — The chief advantage is the abihty to supply lamps and 
self-starting motors from the same circuit or from the same char- 
acter of generator, avoiding a multiplicity of generator types at 
the station. 

Q. 403 — Cannot self-starting motors be operated on simple 
alternating-current lines ? 

A. — Yes; self-starting single-phase motors are in use, but they 
do not give quite so satisfactory results as polyphase machines^ 
especially in larger sizes. Moreover, they require special con- 
struction or auxiliary apparatus to enable them to be self-start- 
ing. A simple single-phase motor, without any special starting 
device, will not move from a dead rest when thrown into circuit. 

Q. 404 — Are two-phase and three-phase motors self-starting? 

A.— Yes. 

Q. 405 — Is there any preference between two-phase and three- 
phase systems? 

A. — The three-phase system is more economical in line wires, 
but a two-phase system is easier to maintain in ''balance," and 
consequently gives better regulation at the generators. 

Q. 406- — Why is the three-phase system more economical in 
wire? 

A. — Because of the phase relations between the three currents 
As explained under Q. 300-303, the three currents rise and fall at 
different instants; the result of this is that the "drop" in a three- 
wire three-phase line is exactly the same that it would be in a 
two-phase line having four wires of the same size as those in the 
three-phase line. 

Q. 407 — How does it compare with a single-phase line? 

A. — Exactly the same way; the amount of copper required in 
a single-phase two-wire line is the same as that required in a two- 



Motor Circuits — Synchronous Motor. i6i 

phase four-wire line for the same load and drop — the two wires 
have each twice the cross-section of each of the four wires of the 
two-phase line. Therefore, the three-phase three-wire line re- 
quires three-fourths the amount of copper for a given set of con- 
ditions that is required by the simple alternating-current line 
with two wires, and also by the four-wire two-phase line. 

Q. 408 — With the voltage between wires the same in all three 
systems, how does the current in each wire compare? 

A. — The actual amount of power transmitted being the same 
in all three cases, the current per wire in a two-phase line is one- 
half that in a single-phase line; the current per wire in a three- 
phase line is 0.577 of the current in a single-phase line, and 1.155 
times the current per wire in a four-wire two-phase line. 

O. 409 — How many different kinds of alternating-current mo- 
tors are there ? 

A. — There are two general classes, known as synchronous and 
induction motors ; each of these is again divided into single-phase 
and polyphase. 

Q. 410 — What is the difference between synchronous and in- 
duction motors ? 

A. — There are two distinctions; a synchronous motor has its 
field excited from some direct-current source^ while its armature 
takes current from the alternating-current line, whereas an induc- 
tion motor field is supplied from the alternating-current circuit 
and its armature is not connected to any source of current, the 
currents in it being induced by the field — hence its name of "in- 
duction" motor. Again, a synchronous motor having a certain 
number of poles and being supplied with current from an alter- 
nator having the same number of poles will run at the same speed 
as that of the alternator, regardless of the load or voltage — hence 
the name "synchronous" motor; on the other hand, an induction 
motor, although tending to run in synchronism with the gen- 
erator which supplies it with current, cannot do so, but lags be- 
hind the generator by a small amount, the actual lag, or "slip" 
as it is termed, varying with the load on the motor. 

Q. 411 — Is either type of motor anything like an alternator? 

A. — Yes; a synchronous motor is precisely like an alternator. 
In fact, the two are interchangeable, exactly as in the case of 
direct-current dynamos and motors. 



1 62 



The Synichronous Motor. 



Q. 412 — And does a synchronous motor require a separate 
exciter, like an alternator? 

A. — Yes. In some cases the motor has an individual exciter, 
either mounted on the end of the motor shaft or belted to it, 
therefore driven by the motor itself; in other cases, a single ex- 
citer dynamo supplies current to the fields of a whole plant or 
group of synchronous motors, just as in the case of alternating- 
current generators. 

Q. 413 — What drives the exciter in the latter case? 

A. — It is driven either by one of the motors or by an individual 
motor devoted solely to that work. 

Q. 414 — Could not a battery be used for exciting the fields? 




FIG. 207. 



A. — Yes, but this is not good practice ordinarily, because of the 
lack of facilities in an alternating-current plant for charging the 
batteries. 

O. 415 — Why does a synchronous motor run at exactly the 
speed of the generator supplying current to the circuit? 

A. — A consideration of the action of a direct-current motor will 
assist in understanding this. If the field of the motor be main- 
tained at fixed polarity, the direction in which the armature turns 
will depend on the direction of the current flow through the arma- 
ture winding. Reversing the current reverses the direction of 
rotation. This, as has been explained previously, is because pass- 
ing a current through a wire lying across a magnetic field causes 
the wire to be dragged in one direction, and reversing the current 



speed of a Synchronous Motor. 



163 



in the wire causes it to be dragged in the other direction. Now, 
the current in the wires on a synchronous motor armature is con- 
stantly reversing, so that if a given group of wdres were held near 
a north magnet pole, say, they would be first attracted toward the 
pole and then repelled from it. But if the wires are held near a 
north pole when the current passes in the right direction to cause 
attraction, and then allowed to move on within reach of the south 
pole just as the current reverses, the attraction would be con- 
tinuous in the same direction. 

Q. 416 — Then each wire must move from one pole to another 
every time the current reverses, in order to be continuously at- 
tracted? 




A. — Exactly. Reference to Figs. 207 and 208 will make this 
still clearer. Here the illustration of magnetic pull is used. With 
current passing through the four armature coils in the directions 
indicated by arrows in Fig. 207, north poles will be formed at n 
and n, and south poles at .? and s. and the armature will be at- 
tracted around m the direction taken by clock hands, until the 
south poles are opposite the north poles of the magnet, as in Fig. 
208. At this point a reversal of the current in the armature coils 
will carry the armature on through another quarter revolution, 
and so on, it being necessary only to reverse the current every 
time an armature "pole" comes opposite a field-magnet pole. 

Q. 417 — How is the motor made to turn at exactly the speed 
necessary to bring the coils from pole to pole in time with the 
reversals of the line current? 



164 Starting' a Synchronous Motor. 

A. — It is run up to that speed by some outside means and then 
connected to the circuit; afterward, the pull between the field 
poles and the armature will keep the speed up, unless the machine 
be overloaded. 

Q. 418 — Then a synchronous motor is not self-starting? 

A. — It is to a certain extent, but the flow of current through the 
armature is extremely heavy during the starting, which is not due 
to direct magnetic pull between the magnet poles and the arma- 
ture poles, but to a complicated reaction between the two which is 
highly inefficient and cannot be explained to anyone who is not 
throughly versed in alternating-current phenomena and mathe- 
matics. 

Q. 419 — What means is usually employed for starting syn- 
chronous motors ? 

A. — A small induction motor is mounted on the shaft or ar- 
ranged to be belted to the synchronous motor shaft during the 
starting-up period. The induction motor is thrown in circuit 
first, and brings the armature of the synchronous machine to a 
speed slightly above synchronism; then the synchronous arma- 
ture is connected to the mains and the induction motor is cut out. 
The field magnet of the synchronous motor is fully excited, of 
course, beforehand. 

Q. 420 — What is the relation between the speed of a synchro- 
nous motor and that of the generator supplying it with current 
when they have different numbers of poles? 

A. — The speed of the motor is that at which it would have to 
run, if driven as a generator, to deliver the number of cycles 
which is given by the supply alternator. For example, a 12-pole 
alternator running at 600 revolutions per minute (10 per second) 
will deliver current at a frequency of 60 cycles a second; an 8-pole 
synchronous motor supplied from that circuit will run at 900 
revolutions per minute, which is the speed at which it would have 
to be driven as a generator to give 60 cycles a second — the fre- 
quency of the 12-pole alternator. 

Q. 421 — Is there a simple formula giving the speed relations 
between generators and motors connected to the same circuit and 
having different numbers of poles? 

A. — Yes; if P represents the number of poles of the generator 



Polyphase Synchronous Motors. 



165 



and 5 represents its speed; and if the poles and speed of the 
motor be represented by p and s, respectively, then 

PXS 

Q. 422 — Is the difference between polyphase and single-phase 
synchronous motors the same as the difference between the gen- 
erators ? 

A. — Of course; any alternator built to supply a certain kind of 
circuit will operate as a synchronous motor if its armature be 





rnmmm 

MOTOR FIELD 



EXCITER 



SINGLE-PHASE. 




EXCITER 



TWO-PHASE. 

FIG. 209. 




EXCITER 



THREE-PHASE 



supplied with current from that kind of a circuit. Consequently 
a two-phase alternator is a two-phase synchronous motor, and so 
on throughout the list of dififerent kinds of alternators. 

Q. 423 — Then the circuit connections of a synchronous motor 
must be the same as those of an alternator, are they not? 

A. — Yes ; Fig. 209 shows the connections. The circles repre- 
sent the collector rings on the armature shaft. 

Q. 424 — What is the dift'erence in construction between a 
synchronous motor and an induction motor? 

A. — There is a great deal. The stationary part of an induction 



1 66 



The Induction Motor. 



motor is not at all like the field magnet of a synchronous motor, 
but is made up of thin disks shaped as shown by Fig. 210, just 
as an armature is made up. The revolving part, or rotor, is also 
radically different from an ordinary armature. It is made up of 




Powe^.y.Tj: 



FIG. 210. 



disks, like an armature core, but instead of slots for the winding, 
it has a series of holes around the edge, as shown by Fig. 211. 




FIG. 211. 



Q. 425 — How are the field coils arranged? 

A. — The field coils are disposed exactly like the armature coils 
of an alternator, so that each coil embraces several teeth, being 
divided into two or more sections, according to the number of 



Sfator JJ^iiidiu^s. 



167 



teeth per pole. Fig. 212 shows diagrammatically the winding for 
a pole of three teeth, for a single-phase motor field, or stator. 
Q. 426 — How are the field windings of a two-phase motor ar- 



ranged ? 




Pouer. X r. 



FIG. 212. 



A. — The winding is divided into two groups, each wound to 
give the same number of poles, but interlinked, as shown by Fig. 
213, so that the center of any pole of one group will be half-way 
between the centers of the adjoining poles of the other group. 
The current in one group of windings is at its maximum value 




Power, X. 7. 



FIG. 213. 



when the current in the other group is at zero, the two currents 
being supplied from a two-phase generator. 

Q. 427 — Then the poles are produced first by one set of wind- 
ings and then by the other, are they not ? 

A. — Xot exactly that; there are two sets of poles produced by 



i68 The Rotor. 

the two ^Yindings, and each set rises to its maximum strength at a 
different instant from the other. The resuhant is what is known 
as a "rotary" field. Tlie change of maximum polarity from one 
set of windings takes place in gradations, not in abrupt jumps, 
so that the effect in the air-gap is precisely the same as though a 
field magnet, excited by direct current, were revolving about the 
armature. 

Q. 428 — If the field revolves, why does the armature revolve 
also? 

A. — The mechanical field structure does not revolve; it is only 
the magnetic field in the air-gap that rotates. This induces cur- 
rents in the rotor, which make it revolve in an effort to keep up 
with the rotating field. ' 



^ "^7' 'Ill' I'll ' i.jir ey. .Y. r. 

FIG. 214. 

Q. 429 — How is the rotor wound? 

A. — In small machines it is not wound at all. The conductors 
are stout copper rods, threaded through the holes in the core 
and joined at both ends by a ring of copper, as shown by Fig. 
214. 

Q. 430 — If the conductors are short-circuited, as the engrav- 
ing shows them, why does the armature not burn out? 

A. — Because the armature so nearly keeps up with the rotating 
field that the number of magnetic lines cut by the rotor con- 
ductors is very small, compared with the total number in the air- 
gap. The voltage induced in the rotor conductors is just suf- 
ficient to force enough current through the rods to pull the load. 

Q. 431 — What regulates the difference between the rotor speed 
and that of the rotating field ? 

A. — The load. If the rotor were driven at exactlv the same 



ludiiction Motor Torque. 169 

•speed as the magnetic field, no current would flow in the rotor 
•conductors, because they would travel with the flux instead of 
-cutting it. Now, if it be left free for the flux to drag it around, 
it will lag behind the rotation of the flux, tending to stop, until 
the conductors cut sufficient lines of force to generate a voltage 
sufficient to force current through the conductors in large enough 
quantity to keep the rotor going. 

O. 432 — But what determines just how much current is 
needed ? 

A. — The load. Work is measured in foot-pounds per minute. 
Now, if the motor is to do one-third of a horse-power (ignoring 
losses), this means 11,000 foot-pounds per minute; if the circum- 
ference of the rotor moves at the rate of 1,100 feet a minute, there 
must be a pull of 10 pounds on the conductors, exerted by the 
rotating flux. As explained under Q. 151- 154. the pull in pounds 
between a conductor and a magnetic flux is given by the formula 

= Pull in poundSf 
Ki- 
rn which formula (J) represents the magnetic flux cut per revolu- 
tion by the conductor; C is the current in the conductor, K, a 
constant and r. the radius of the rotor, in feet. As already ex- 
plained, the more the rotor lags behind the magnetic field, the 
more lines of force are cut by the conductors; consequently, the 
more E.M.F. is induced in them and the more current is forced 
through them by this E.:\I F. Therefore, the rotor speed will 
drop back until the number of magnetic Hues cut by the con- 
ductors and the current passing through them are just sufficient 
to give the requisite pull; it cannot drop below this, because the 
pull keeps it up, but if there is the least increase in the load, re- 
quiring more pull, the speed will fall off a trifle until the balance 
in forces is restored. 

Q. 433 — The speed must be very unsteady, is it not? 

A. — Not more so than in a good shunt-wound direct-current 
motor. It must be remembered that the pull increases just as fast 
as the speed changes, because an increase in magnetic lines also 
gives an increase in current. Thus, if the speed slacks ofif enough 
"to double the number of magnetic lines cut per second, the pull 
will be doubled because the E.M.F. will have been doubled, 
•causing the current to double also. 



I/O 



Tzco-Phase and Thrce-Phase Windings. 



O. 434 — If the armature or rotor is not connected to the cir- 
cuit the motor does not give any counter E.M.F., does it? 

A. — Yes. The rotating field flux sweeps all of the wires around 
the inner edge of the stator ring, just as the flux from a rotating 
field magnet in a direct-current generator sweeps the wires oa 
the stationary armature, and induces a counter E.M.F. in the 
field or stator windings. Fig. 215* is an end view — a sort of dia- 
grammatic end view — of a two-phase four-pole induction motor,, 
and Fig. 216 is an axial section of the same machine. 

Q. 435 — What is the difference between a two-phase motor 
and a three-phase motor? 

A. — Simply the division of the field or stator coils into three 




COPYRIGHT, 1900, BY 8P0N A CHAMBERLAIN, N.Y. 



Power, N.T. 



FIG. 215. 



FIG. 216. 



equal groups instead of two, the coils being put on in a manner 
corresponding precisely with the arrangement of three-phase 
alternator armature coils shown in a previous number. Fig. 217 
shows a half view of a three-phase stator with the coils in place. 

Q. 436 — Does the field pass from one set of windings to the 
other^ as in a tw^o-phase motor? 

A. — Yes; the rotation is more uniform, because each cycle is 
divided into three parts instead of two — that is to say, the rise 
and fall of the three separate fields is more gradually interlinkedl 
than when there are only two. 



* From "Polyphase Electric Currents," by S. P. Thompson. 



Induction Motor Speed. 



171 



Q. 437 — How is the speed of an induction motor calculated? 
A. — The speed which it would take if the motor ran exactly 
with the rotating field is usually the one given. This is found 




FIG. 217. 



by dividing the frequency of the current supply by one-half the 

number of poles per phase. In formula shape: 

2 X Frequency 

=— ; = Revs, per second. 

Poles 



1/2 Polyphase Windings. 

It should be remembered that the term ''poles," as applied to an 
induction motor, always means the number of poles per phase, 
or per winding. (Each winding is connected to a separate phase 
of the supply circuit.) 

Q. 438 — Then there are as many windings as there are phases? 

A. — Always; and the field windings are divided up to form the 
same number of poles each, whether the machine is a two-phase 
or a three-phase motor. 



CHAPTER XIL 

THE ROTARY CONVERTER— REACTIVE 
REGULATORS. 

Q. 439 — What other alternating-current machinery is there in 
use? 

A. — In the machinery class there is no more apparatus that can 
be considered as belonging strictly in the domain of alternating 
currents. Rotary converters are usually referred to as alternat- 
ing-current apparatus because they are used in connection with 
alternating-current plants; they form the connecting link, how- 
ever, between the two great classes of alternating and direct-cur- 
rent machinery. 

Q. 440 — What is a rotary converter? 

A. — Broadly speaking, it is a direct-current dynamo (or motor) 
provided with collector rings and brushes in addition to the usual 
commutator and brushes. 

Q. 441 — What work does it do? 

A. — It is used either to change alternating currents into a 
direct current or the reverse; usually it converts from alternating 
to direct current. Current is supplied to the armature through 
the collector rings, and the machine runs as a synchronous motor; 
the field magnet is excited from the brushes on the commutator 
which deliver direct current. 

Q. 442 — If the current that goes into the armature is alter- 
nating, how can it deliver direct current at the other brushes ? 

A. — The current is rectified by the passage of the armature 
wires under the field magnet poles, in a similar manner to the 
operation of a simple rectifying commutator. A reference to O. 
269 and 270 will show that if it were not for the commutator 
of a direct-current dynamo, it would deliver alternating current ; 
in other words, the kind of current supplied by the armature of 



1/4 



Converter Armature Connections. 



an ordinary dynamo depends entirely upon the kind of apparatus 
employed for delivering the current to the outside circuit. If a 
commutator is used, we get direct current, but if collector rings 
are used, connected to opposite points of the winding of a bi- 
polar armature, as in Fig. 218, and the field magnet is excited by 
direct current, we will get alternating current at the brushes. 
Xow, the rotary converter is provided with both the collector rings 
and the commutator, so that it can deliver either direct current or 
alternating current, if it be driven by belt as a dynamo, or it may 
even deliver both at once. These machines are used in this way 
occasionally, in which case they are called double-current gen- 
erators. 

Q. 443. — Still it is not clear how the alternating current deliv- 
ered to the collector rings of a rotary converter comes out as direct 
current at the commutator. 




FIG. 218. 



A. — The current generated in the armature when it is driven by 
a belt is alternating current while it is in the armature ; it is 
changed to direct current by the commutator. Similarly, the 
alternating current fed to the winding through the collector rings 
traverses that winding precisely as though it were generated in 
it, and is changed to direct current by the commutator, with rela- 
tion to the outside circuit to which it is delivered. It remains 
alternating current within the armature winding. 

Q. 444. — And in passing through, it drives the armature as a 
motor ? 

A. — Precisely. And the operation is reversible ; tne machine 
can be run as a motor on a direct-current circuit and alternating 
current may be taken off at the collector rings, exactly as from an 
alternator. 



Converter Aruiature Connections. 



175 



O. 445. — Is a rotary converter multipolar? 

A. — Always. Each of the collector rings is connected in actual 
practice to one-half as many points in the winding as there are 
poles, and the points must be precisely equidistant around the 
winding for each ring; in a single-phase converter there are two 
collector rings, and the points to which each is connected lie ex- 
actly half-way between the points to which the other ring is con- 
nected, as indicated in Fig, 219. 

Q. 446. — How are two-phase and three-phase armatures con- 
nected to the collector rings ? 

A. — Each ring is connected to half as many points as there are 
magnet poles, and the points are at equal distances, as before ; in 




Power, N. 7. 



FIG. 219. 



a two-phase machine there are four collector rings, and therefore 
four sets of taps or "points," and the four are equally spaced, 
exactly like the individual sets of taps. In a three-phase machine 
there are three rings and three sets of taps, the three sets being 
equally spaced with relation to each other, and the individual taps 
of each set being similarly placed with relation to each other. Fig. 
220 is a diagram of the two-phase connections and Fig. 221 is a 
similar diagram of the three-phase arrangement, a four-pole field 
magnet being shown for sake of simplicity. These and Fig. 219 
show the winding as a sort of Gramme ring, but it makes no 
difference whether it is a ring or a drum winding, so long as it is 
a symmetrical, closed-circuit arrangement of coils. In practice 



176 



Rotary Converter Regulation. 



the ordinary drum winding is used. In the diagrams the phases- 
are indicated by the letters A, B and C 

Q. 447. — What determines the speed of a rotary converter? 

A. — If it converts from alternating to direct, its speed is de- 
termined by the frequency of the alternating-current circuit, ex- 
actly like that of a synchronous motor. If it converts in the other 
direction, the speed is determined by the voltage of the direct- 
current circuit, like an ordinary direct-current motor. 

O. 448. — How is the machine regulated ? 

A. — When converting from alternating to direct current, the 
voltage of the direct-current output is regulated by varying the 




FKJ. 220. 



voltage of the alternating current supplied to the machine. This 
is usually done by means of an adjustable reactive coil in series 
with the supply circuits. When converting from direct to alter- 
nating current, the voltage of the delivered alternating current is 
regulated by means of adjustable reactive coils, in the same way 
as before. 

Q. 449. — Why cannot the voltage be regulated by varying the 
strength of the field magnet ? 

A. — Because, as there is only one winding on the armature, 
there is an absolutely fixed ratio between the arithmetical average 
of the voltage of all the coils (direct current) and the geometrical 
average of the voltage of the coils between the taps. If the ma- 



Converted Voltages. 



^77 



chine is delivering direct current, the voltage of that current is 
proportional to that of the alternating current supplied to the 
collector rings, regardless of the strength of the field; thus, a 
single-phase converter supplied with alternating current at lOO 
volts will deliver only 141. 4 volts at the direct-current commu- 
tator; a two-phase machine will deliver the same voltage, and a 
three-phase machine will deliver direct current at 163.3 volts. 
Conversely, if it converts from direct to alternating, each 100 volts 
delivered to the commutator will cause a single-phase or two- 
phase machine to deliver 70.7 volts alternating, and a three-phase 
machine to deliver 61.23 volts alternating. 




FIG. 221. 



Q. 450. — What is the adjustable reactive coil mentioned above? 

A. — It is a laminated core magnetized by a primary winding, 
like a transformer, and having secondary coils connected in series 
with the circuit between the transformer secondaries and the 
collector rings of the rotary converter; the effect of the coils is 
adjustable, so that the secondary E.M.F. may be varied to suit 
the requirements of the service. Fig. 222 is a diagram illustrating 
the general connections. 

Q. 451. — How is the effect of the regulating coils adjusted? 

A. — There are several ways. Fig. 223 shows the principle of 
a single inductive regulator. The fine-wire coil, P, is connected 
across the secondary circuit and magnetizes the laminated iron 



178 



Reactive Regulators. 



core, C ; an E.M.F. is thereby induced in the coarse-wire coil, S, 
which is in series with the circuit. The core is pivoted at its cen- 
ter so that it may be turned to the position shown in Fig. 224 ; in 
one position the E.M.F. induced by the secondary coil boosts the 
circuit, and in the other it opposes it. 




CONVERTER 



REGULATOR 

FIG. 222. 



0. 452. — Cannot the amount of boosting and opposition be 

aded? 

A. — Certainly, when the core is parallel with the coil, S, the 



graded 




FIG. 223. 

regulator neither boosts nor opposes ; but a slight movement in 
the one direction results in a slight boosting, which increases very 
gradually until the core is midway between the coils, as in Fig. 
223, for example. Conversely, a slight movement from the neutral 
position (within the coil S) m the opposite direction gives a small 



Reactive Regulators. 



179 



opposing E.M.F., which increases until the core is midway be- 
tween coils on the other side, as in Fig. 224, for example. 

Q. 453. — Are there any other forms of induction regulator ? 

A. — Fig. 225 represents an arrangement that has been used to 




FIG. 224. 

some extent. The principle is precisely the same — the boosting 
or lowering is regulated by varying the inductive relation between 
one of the windings and the rotatable core, C, and the adjustment 
gives a smooth gradation from maximum opposition to maximum 
boosting. With the core turned to the position shown in Fig. 226, 
the secondary winding has no E.M.F. induced in it. 

Q. 454. — How do the two forms compare ? 

A. — There is not much choice between them. The one in Fig. 
223 is less expensive in construction, and has the advantage that 
the windings are stationary. The one in Fig. 225 has also the 




FIG. 225. 



disadvantage that the coarse-wire winding is the movable one, 
rendering heavy flexible connections necessary. Fig. 227 repre- 
sents a regulator devised by the writer several years ago, in which 
this disadvantage is obviated and the coils of which are more 



i8o 



Reactive Voltage Regulators. 



efficient inductively. This regulator, however, embodies the dis- 
advantage of a moving high-potential coil and dangling high- 
potential connecting leads. In the position here shown, there is 
no E.M.F. induced in the secondary coil, S, but its reactance will 




cut down the circuit E.M.F. slightly ; the neutral position of the 
core, therefore, would be with the primary coil tilted somewhat 
to the right or left (according to the connections) of the position 
shown. 



CHAPTER XIII. 
ELECTRIC LIGHT —THE ARC LAMP. 

Q. 455 — How is light produced by electricity? 

A. — There are two methods in common use, and a third is in 
process of development. Those in use are the vaporization and 
combustion of carbon, in the electric arc, and bringing a fine car- 




bon thread to a white heat, in the incandescent lamp. Fig. 228 
shows the appearance of an electric arc produced by direct current. 

Q. 456 — How does the vaporization of carbon produce light? 

A. — It does not, directly ; the ends of two sticks of carbon are 
brought together and a current is passed through them; then 



i82 The Electric Arc. 

they are separated and the current continues to flow across the 
gap between them by virtue of a so-called ''stream" of carbon 
vapor, which serves as a conductor between the two points, and 
also as a fuel for combustion to enhance the light. 

Q. 457 — What causes the carbon vapor stream? 

A. — The first passage of current from carbon to carbon heats 
the carbon tips and volatilizes them. 

Q. 458 — Can an arc be maintained between metal tips ? 

A. — Not with ordinary voltages, and a steady arc cannot be so 
maintained in open air at any voltage. 

Q. 459 — What voltage is required for an arc between carbon 
points ? 

A. — If the arc is operated in the open air, from 42 to 55 volts 
are required at the gap between the carbons with direct current 
and 28 to 35 volts with alternating current, according to the length 
of the arc and amount of current passing. 

Q. 460 — -Why is less voltage required with alternating current ? 

A. — Because a steady arc cannot be maintained across so long a 
gap as with direct current, and the shorter gap requires less 
E.M.F. to force the current across. 

Q. 461 — What causes the difference in steadiness ? 

A. — With direct current there is an electrolytic wasting away 
in the center of the positive carbon, forming a hollow or crater in 
the end; this crater steadies the arc. With alternating current 
no crater is formed, because both carbons are alternately positive 
and negative, preventing any electrolytic action. Both carbons 
burn to a point, and the arc becomes unsteady and flickering at 
a shorter length than when steadied by the direct-current crater. 

Q. 462 — Is an arc operated otherwise than in open air? 

A. — Yes ; the most modern form of lamp is provided with a 
glass envelope or inner globe immediately surrounding the arc, 
as shown by Fig. 229. 

Q. 463 — What is the object of enclosing the arc? 

A. — To prolong the life of the carbons, and also to obtain a 
more diffused light. 

Q. 464 — How is the life of the carbons prolonged ? 

A. — The supply of oxygen is greatly restricted, and the rate of 
consumption is therefore slower. The envelope cannot fit abso- 



Life of Open-Arc Carbons. 



183 



lutely air tight, so that there is still a small amount of oxygen 
supplied to the arc. 

Q. 465 — What is the life of the carbons of an open arc ? 

A. — The life varies with the hardness and size of the carbons 
and the amount of current. With average carbons, \ inch in 
diameter, and a current of 9.6 amperes, the rate of consumption 
is approximately i-| inches an hour for the positive carbon, and a 




FIG. 229. 



trifle over half that rate for the negative carbon, with direct cur- 
rent. 

Q. 466 — Why does the positive carbon burn twice as fast as the 
negative carbon ? 

A. — Partly because the current in passing from the positive 
and to the negative carbon deposits on the negative tip some par- 
ticles of carbon, due to disintegration of the positive tip. More- 
over, in addition to the consumption of the outer part of the posi- 
tive tip in the ordinary manner by oxidation, the center wastes 



184 Life of Enclosed-Arc Carbons. 

away electrolytically, as previously stated, whereas the negative 
carbon is consumed by oxidation alone. 

O. 467 — What voltage is required for an arc enclosed as in 
Fig. 229? 

A. — From 65 to 80 volts, according to the distance apart of 
the carbons and character of the current. 

Q. 468 — Cannot an enclosed arc be operated at the same voltage 
as an open arc ? 

A. — Not satisfactorily. The rate of combustion (oxidation) is 
so slow that a short arc would not give sufficient illumination. 

Q. 469 — What is the life of the carbons of an enclosed arc? 

A. — A first-class grade of carbon under average conditions will 
be consumed at a rate slightly less than iV inch per hour for the 
positive and somewhat more than one-half of this for the nega- 
tive, in a direct-current lamp. 

O. 470 — Is there any difference between the action of the car- 
bons on a direct-current circuit and on an alternating-current 
circuit ? 

A. — A decided dift'erence. With alternating current both car- 
bons burn alike from oxidation only, no electrolytic wasting oc- 
curring. The upper carbon burns a trifle more rapidly than the 
lower because of the ascent of heat from the arc. Furthermore, 
there being no electrolytic waste, there is no hollow or crater 
formed in the center of the upper carbon, as in a direct-current 
lamp; both carbons burn to a point, being consumed from the 
outside by oxidation, as stated under Q. 461. 

Q. 471 — What is the shape of the carbon? 

A. — It is made up into round pencils or rods, from f inch to 
} inch in diameter, and from 7 to 12 inches in length ; the arc is 
formed between the ends of two carbons, as indicated in Fig. 228. 

Q. 472 — What happens when the ends burn away ? 

A. — One of the carbons is automatically moved toward the other 
by minute degrees as the carbon wastes away, so that the length 
of the arc is kept practically constant. This operation is termed 
^'feeding." 

Q. 473 — How is the carbon fed ? 

A. — By some one of several forms of mechanism controlled by 
the current. Fig. 230 illustrates the principle on which many 
lamps are constructed. The carbons are mounted vertically, the 



Differential Clutch Mechanism. 



185 



lower one being held in a stationary clamp and the upper one in 
a clamp at the end of a sliding brass rod, r. This rod is raised 
by a clutch, represented conventionally by the washer c. The 
action is as follows : The upper carbon rests on the lower one 
when the lamp is idle; when current is turned on, it passes 
through a coarse wire solenoid, S, the finger or brush, h, and both 
carbons. Practically no current passes through the fine wire 




FIG. 230. 



solenoid, Sh, as it is short-circuited by the carbons. The solenoid, 
Sy lifts the carbon-carrying rod, r, through the lever, I, and the 
clutch, c, and "strikes" the arc. As soon as the carbons are sep- 
arated, current is shunted through the coil, Sh, and as the carbons 
burn away, more and more current passes through this coil imtil it 
overcomes the pull of the lower solenoid and lowers the clutch 
against the tripping lug, t, relieving the grip of the clutch and 
allowing the rod to feed downward. 



1 86 Feeding. 

Q. 474 — Does the carbon feed down into contact with the lower 
one again ? 

A. — No; the instant the rod begins to feed, the shortening of 
the arc reduces the current in the shunt solenoid, Sh, and the 
lower solenoid immediately renews the grip of the clutch on the 
rod. 

Q. 475 — Then the arc lengthens and shortens ? 

A. — It does to an imperceptible extent; the variations are ex- 
tremely small. For example, when the arc is at normal length, 
say IOC mils,"^ the relations are such that the two solenoids are 
so nearly of equal strength that the weight of the carbon and rod 
is just supported by the lower solenoid. Now, when the upper 
carbon burns away, say 2 per cent (2 mils), the resistance of the 
arc will increase 2 per cent, the voltage between the carbons (and 
consequently at the terminals of the shunt solenoid) will increase 
2 per cent, and the current in the shunt solenoid will accordingly 
increase 2 per cent. This is usually enough to overcome the fric- 
tion of the mechanism and allow the clutch to loosen its grip on the 
rod. As soon as the arc is restored to its proper length the state 
of magnetic equilibrium is restored, and the shunt solenoid re- 
strains the rod from further movement. 

Q. 476 — Why does the voltage at the arc increase when the 
arc lengthens ? 

A. — Lamps of the kind described are operated in series on 
constant-current circuits, and in order to maintain the current 
constant the voltage must be varied at the dynamo as the resist- 
ance of the circuit varies. 

O. 477 — Are any other forms of arc lamp mechanism used on 
constant-current circuits ? 

A. — There are several modifications of the clutch type of 
mechanism, and also a clock-work form in which gearing meshed 
with a rack on the rod is employed to control the movement of 
the rod. Fig. 231 illustrates the principle of the clock-work lamp ; 
here the lower solenoid is in shunt to the arc and the upper one 
in series, the lever being pivoted at the end instead of in the 
middle as in Fig. 230. 

Q. 478 — Are two solenoids always necessary? 



*A mil is 1-1000 inch. 



ShiDit Lamp Mechanism. 



187 



A. — Xo. In many lamps built to operate on constant-potential 
circuits only a single solenoid, in shunt to the arc, is used ; its 
function is to "feed" the carbon. 

Q. 479 — What lifts the carbon to form the arc ? 

A. — The carbons are normally held apart by a spring, and the 
shunt solenoid opposes the spring, tending to draw the mechanism 
downward, as indicated in Fig. 232. When the lamp is connected 
to the circuit, no current passes through the carbons ; as the sup- 
ply circuit is at constant potential, a strong current will pass 
through the shunt coil, pulling down the lever and allowing the 
upper carbon to drop. As soon as it touches the lower carbon, 
the shunt coil is short-circuited and allows the spring to raise the 
clutch and carbon, striking the arc. Thereafter, the spring fulfils 
the function of the series solenoid in the double solenoid lamp. 




Power, y. y. 



FIG. 231. 



O. 480 — When the carbons separate, removing the short cir- 
cuit from the solenoid, does not the line current cause it to pull 
the lever down again, producing a see-saw movement ? 

A. — Xo. When the carbons separate to the normal arc length, 
5 amperes or more flow through them and the resistance coil, and 
the drop in the resistance coil reduces the voltage at the arc below 
the voltage of the circuit, so that the current in the shunt coil is 
less after the arc is established than before. 

Q. 481 — If the circuit is at constant potential, does not the volt- 
age of the shunt solenoid terminals remain constant after the 
arc is struck ? 

A. — X'o ; as the carbons burn away the lengthened arc reduces 
the current through the carbons and the resistance coil. This 
reduces the drop in the resistance coil, leaving more of the circuit 
potential active at the shunt coil terminals. Thus, if the circuit 



1 88 



Constant Potential Lamp. 



E.M.F. were 55 volts, the resistance coil were of 4 ohms and the 
main current at normal arc were 6 amperes, the drop in voltage 
in the resistance coil would be 6 X 4 = 24 volts, leaving 31 volts 
at the arc and the shunt coil terminals. Now, when the carbon 
burns away enough to reduce the current to, say, 5.9 amperes, 
the drop in the resistance coil will be 5.9 X 4 = 23.6 volts, leav- 
ing 31.4 volts at the arc and the shunt coil terminals; the current 




^ 



ZT 



Power, X.r. 

FIG. 2S2. 



in the shunt coil will then be ^-^ = 1.013 times its former 

strength. An increase of from 2 per cent to 5 per cent is 
required to overcome the friction of the mechanism and cause 
feeding. 

Q. 482 — What happens when the carbons are all burned out ? 

A. — If the lamp is operated singly on a constant-potential cir- 



Automatic Ciit-Outs. 



189 



cuit, the rod comes to a stop at the end of its travel, and the arc 
grows longer and longer until it breaks. In a lamp operated in 
series with other lamps, the entire lamp mechanism is short- 
circuited by an automatic cut-out within the lamp box. 

Q. 483 — What is the automatic cut-out like ? 

A. — There are several forms, depending on the type of lamp 
and individual ideas of the makers. Almost all series arc lamps 
contain a contact finger mounted on the carbon-rod and arranged 
to connect the two binding-posts of the lamp when the carbon-rod 
reaches its lowest position. Such an arrangement is indicated in 




Power, A'. Y. 



FIG. 233. 




Fig. 233 ; the series solenoid is in this case connected between the 
bottom carbon and the negative post. 

Q. 484 — What other forms are commonly used? 

A. — There is always some form of emergency cut-out which 
operates in case the mechanism becomes disabled during the life 
of the carbons and also when they have burned out. Fig. 234 
represents an electromagnetic cut-out, the essentials of which 
form the basis of almost all emergency cut-outs. The magnet, C, 
has two windings, one of fine wire connected in shunt to the arc, 
and the other of coarse wire, normally open. As long as the arc 



190 Alternating-Current Lamp. 

is normal in length, the spring, s, is stronger than the magnetism 
due to the fine wire coil. If the arc becomes abnormally long, 
the voltage at the terminals of the cut-out coil rises and the re- 
sultant increase in current in the coil overcomes the spring and 
pulls the armature against the contact. This gives the line cur- 
rent a short path from one post to the other, through the coarse 
wire on the magnet, and a resistance coil, R, cutting out the lamp. 

Q. 485 — Why is the coarse-wire winding used ? 

A. — Because the armature would not stay closed without it, the 
shunt coil being short-circuited along with the other parts of 
the lamp. 

Q. 486 — What is the object in using the resistance coil, Rf 

A. — To maintain an appreciable difference of potential at the 
terminal binding-posts of the lamp so that in a case where the 
cut-out operates on account of a "hang-up" of the mechanism, if 
the trouble should disappear and the carbons come together there 
will be sufficient current shunted through the carbons and series 
solenoid, S, to weaken the series winding of the cut-out magnet 
and open the cut-out, permitting the lamp to resume operation. 

Q. 487 — Why should the mechanism hang up ? 

A. — There are many accidental derangements, any one of which 
might cause the mechanism to stick. If the carbon-carrying rod 
of a clutch lamp is not properly cleaned it may stick in the clutch ; 
the same is true of a rod slightly bent and of one that has been 
dented. A sharp jar might suffice to free the rod and the lamp 
would start up again. 

Q. 488 — Are arc lamps much used on alternating-current cir- 
cuits ? 

A. — Yes, to a great extent. All alternating-current lamps are 
of the enclosed-arc type. 

Q. 489 — Are they exactly like enclosed-arc direct-current 
lamps ? 

A. — In general principle, yes ; in constructional details they are 
different. The chief difference lies in the solenoid core, which is 
laminated in order to prevent ''eddy" currents. 

O. 490 — How can a round rod be laminated ? 

A. — The core is not a rod in such a case and not always round ; 
it is made of either a strip of thin iron rolled into the shape indi- 
cated by Fig. 235 or a bundle of very small iron wires, as in Fig. 



Alternating- Current Solenoid Cores. 



191 



236, or else a bundle of flat iron strips, as in Fig. 237. The last 
two are the commonest forms. 

Q. 491 — Are alternating-current lamps operated in series? 




k 



FIG. 23b. 




FIG. 236. 



FIG. 237. 



A. — Usually, although there are many lamps operated singly 
on constant-potential circuits of 100 to no volts. The lamps that 
work in series are also supplied from some constant-potential 
source but of high E.M.F., the current through the lamps being 
maintained constant by some auxiliary means. 



-o E a 



FIG. 238. 



Q 4C)2 — If the E.M.F is constant how can the current be kept 
constant? 

A. — One of the commonest methods is to insert more or less 



192 



Regulating Series A. C. Circuits. 



reactance in the circuit., so that the total E.M.F. required is always 
the same. For example, in Fig. 238, where each cross mark 
represent a lamp, if each lamp requires 65 volts and 6.6 amperes 
and the drop in the circuit wiring is 50 volts, the E.M.F. required 
at the source, E, would be 30 X 65 -f- 50 == 2,000 volts. Now if 
two lamps be cut out and a reactance of 19.7 ohms be inserted, 
as indicated in Fig. 239, the E.M.F. required by the lamps and 
wire will be reduced to 28 X 65 + 50 = 1*870 volts; but the 
drop at the reactance will be 19.7 X 6.6 = 130 volts, so that the 
total E.M.F. required at E will remain 2,000. 

Q. 493 — Then the reactance must be adjusted every time a 
lamp is cut in or out? 

O E O 




JOO. 



FIG. 239. 



A. — Yes; but this is done automatically. A regulator is used 
which varies the reactance as the circuit requires it, without any 
attention whatever. 

O. 494 — How is the regulator constructed? 

A. — Fig. 240 illustrates the arrangement of one of these instru- 
ments. Two solenoids, M M, are hung from an insulating slab 
and a U-shaped core, C, is suspended from one arm of a lever, L, 
with the ends of its legs inserted a little way into the coils, as 
shown. The core is partly counterbalanced by weights, W. So 
long as the line current remains normal, with the full number of 
lamps in circuit, the ends of the core legs remain near the ends of 
the coils, the weights being adjusted to balance the core and the 
magnetic pull of the coils under those conditions. Now if one or 



Automatic Adjustable Reactance Coil. 



193 



more lamps be cut out, lowering the resistance of the circuit, the 
current will increase, and the solenoids being strengthened, they 
will draw the core upward. As it enters the coils, the core in- 
creased their self-induction and consequently their reactance, and 
the increase in reactance reduces the current. When the core has 
been drawn up to such a point that the current has decreased to 
normal, the pull of the solenoids is exactly balanced by that part 
of the core weight not counterbalanced by the weights, and no 




FIG. 240. 



further movement occurs. Cutting out more lamps causes a fur- 
ther movement upward, while cutting in lamps weakens the line 
current and the core drops until equilibrium is restored. A dash- 
pot below the core steadies its motion and prevents see-sawing 
when there happen to be several changes in the line in rapid suc- 
cession. 

Q. 495 — Is not the pull of the solenoid greater when the core is 
far in than when it is at the mouth ? 

A. — Yes; with the core half way up the solenoid, the same 



194 



Automatic Series Circuit Regulator. 



amount of current will exert a stronger pull than when it is at the- 
bottom., 

Q. 496 — Then will not the core require less current to balance 
it the higher it goes ? 

A. — It would except for the shape of the lever arm to which 
the counterweights are hung. As the core goes up and the 
weights go down their effective pull decreases, owing to the con- 
stantly decreasing "leverage," so that the solenoid has to hold up 
a greater and greater proportion of the weight of the core. The 




FIG. 241. 



angle of the weight arm is such that the decrease in effective 
counterbalancing as the weights go down corresponds precisely 
to the increase in the pull of the solenoids at normal current. 

Q. 497 — Why cannot the circuit E.M.F. be adjusted, as in the 
case of a direct-current arc circuit ? 

A. — It can be, and this method is extensively used. Fig. 241 
represents one form of automatic potential regulator. The trans- 
former that supplies current to the arc circuit has taps led out 
from the secondary winding by means of which any proportion 



Constant-Current Transformer. 



195 



of the total winding may be connected to the circuit. Two sole- 
noids, / and D, actuate a pivoted arm which cuts in and out sec- 
tions of the winding just like a rheostat arm cuts in and out re- 
sistance wire. A relay in the lamp circuit sends current through 
one or the other of the solenoids, according to the requirements 
of the circuit. The solenoid / increases the impressed E.M.F. by 
cutting in sections of the transformer secondary, and D works in 
the opposite direction. 

Q. 498 — Are there any other methods of regulating series alter- 
nating current circuits ? 

A. — One other method is in general use. This consists of using 
a transformer which takes current at constant potential in its 




P-jU'er, N.r 



FIG. 242. 



primary winding, and delivers constant current at varying E.M.F. 
at the secondary terminals. 

O. 499 — How does this transformer differ from the ordinary 
kind? 

A. — It is provided with movable secondary coils that are shifted 
bodily away from or toward the primary winding, according as 
the secondary E.M.F. needs to be decreased or increased in order 
to maintain constant current. 

Q. 500 — What shifts the coils ? 

A. — The mutual magnetic repulsion between the primary and 
secondary windings. Fig. 242 shows the principle of construction. 
The primary coil, P, is stationary, and the secondary coil, S, is 
hung from levers carrying weights that partly counterbalance it. 
As long as the current remains normal, the force of repulsion 



196 Constant-Current Transformer. 

between the coils equals the uncounterbalanced part of the coil's 
weight. Any increase in secondary current increases the repul- 
sion, and the coil is lifted away from the primary coil until the 
E.M.F. induced in it by the primary has decreased enough to 
bring the secondary current down to normal value. 

Q. 501 — Why does the distance between the coils affect the 
secondary E.M.F. ? 

A. — The greater the distance the less primary magnetic flux 
will be cut by the secondary wires. 

Q. 502 — What becomes of the flux that is not cut? 

A.— It leaks around the outside of the primary coil, between 
the two windings. 

Q. 503 — Why do the two coils repel each other? 

A. — Because the current in the secondary is always opposite 
in direction to that in the primary, so that at any given instant 
the flux due to the secondary coil is opposed to that created by the 
primary coil. 



CHAPTER XIV- 
THE INCANDESCENT LAMR 

Q. 504 — What is the construction of the incandescent lamp? 

A. — A fine thread of carbon, known as a "filament," is bent into 
one or more loops or spirals and cemented at the ends to two 
pieces of platinum wire; the whole is enclosed in a glass bulb, 




FIG. 243. 

from which practically all of the air has been exhausted, the 
platinum wires projecting through the wall of the bulb and con- 
necting with the outside lamp terminals. Fig. 243 shows a mod- 
ern incandescent lamp. 

Q. 505 — Why does this filament give light? 

A. — The passage of current through it raises its temperature 
to white heat. 



198 Life and EfHcIency. 

Q. 506 — How much current is required? 

A. — That depends on the thickness and character of the fila- 
ment. The current ranges from 1-6 ampere in lamps of high 
voltage and small power to 10 amperes in lamps of low voltage 
and large size. 

Q. 507 — Why is platinum wire used to connect the filament 
with the lamp terminals? 

A. — Because its coefficient of expansion is so near that of the 
glass that expansion and contraction do not affect the seal where 
the wires go through. This is not true of any other metal. 

Q. 508 — Why is the air exhausted from the bulb? 

A. — In order to prevent the immediate destruction of the fila- 
ment by combustion. A lamp filament heated even to a red 
glow in air will be promptly consumed. 

Q. 509 — Does the filament last indefinitely in a vacuum? 

A. — No. The useful life ranges from 400 to about 1,000 hours, 
according to the efificiency of the lamp. 

Q. 510 — What is meant by the efficiency? 

A. — The candle power given out* per watt of power delivered 
to the lamp. Thus, a i6-c. p. lamp, taking 64 watts, and known 
as a "4-watt" lamp (4 watts per candle), has an efficiency of i 
candle per watt. 

Q. 511 — How does the efficiency affect the hfe? 

A. — The higher the efficiency the smaller the filament and the 
greater the current density in the filament. The higher, there- 
fore, will be the temperature, and, consequently, the more rapid 
the deterioration of the filament. 

Q. 512 — What is the ordinary temperature of a filament? 

A. — According to Prof. H. J. Weber, 2895.8° Fah. for a 3.1-watt 
filament and 2840° Fah. for a 4-watt filament. 

Q. 513 — Why does the filament deteriorate if there is no com- 
bustion ? 

A. — Because of a sort of wasting away, due to what appears to 
be the vaporization of very minute particles of carbon from the 
surface of the filament, which are deposited on the wall of the 
bulb. This wasting or ''evaporation" is uneven, the filament be- 



* The mean horizontal candle-power, measured with the lamp revolving at 180 r. p. ra. 



Filament Deterioration. 



199 



comes thinner at some one point than elsewhere, and this point 
gives way. 

Q. 514 — Does not this wasting affect the Hght before the fila- 
ment breaks? 

A. — Very materially. The reduction in the size of the filament 
reduces its radiating surface, and also increases its resistance so 
that less current passes and less light is produced. Moreover, 
the deposit of carbon on the interior of the bulb reduces more and 
more the passage of light rays as the deposit increases. The re- 












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FIG. 244. 



duction of candle-power due to these two causes usually reaches a 
point, before the filament breaks, where it is more economical to 
buy a new lamp than to continue using the old one. 

Q. 5^5 — How rapidly does the ordinary lamp deteriorate? 

A. — This is easiest shown by a diagram, such as Fig. 2/^4, 
where the ''curve" across the regular rulings indicates the candle- 
power of a well-known 3.1-watt, i6-c. p. lamp at different points 
during its life. Starting at 16.1 candle-pa wer, the brilliancy in- 
creases abruptly to 17.2 candle-power and then gradually falls, 
until after 600 hours it has dropped to 14.15 candles. 



200 



Eifect of Abnormal Voltage. 



Q. 516 — What are the standard efficiencies? 

A. — Lamps for no volts are made to give a candle power with 
3.1, 3^ and 4 watts; lamps for 50 volts and under have been 
made to take as low as 2.8 watts per candle; while 220-volt lamps 
usually take from 4 to 4J watts per candle. 

Q. 517 — What is the resistance of a filament? 

A. — The resistance of a 3^-watt, i6-c. p. lamp for 112 volts is 
precisely -J ohm, hot; when cold the resistance is about i ohm. 



STANDARD 8,1 WATT LAMP. 


STANDARD 3.5 WATT LAMP. 


Voltage 
Per Cent 

of 
Normal. 


Candle- 
power 
Per Cent 
of 


Watts 

per 
Candle- 


Life Per 
Cent of 

Normal. 


Deteri- 
oration 
Per Cent 
of 


Candle- 
power 
Per Cent 
of 


Watts 

per 
Candle- 


Life Per 
Cent of 

Normal. 


Deteri- 
oration 
Per Cent 
of 


Normal, 


power. 




Normal. 


Normal. 


power. 




Normal, 


90 


54 


4.63 


941 


11 


53 


5.36 






91 


38 


4.41 


716 


14 


56 


5.09 






92 


£2 


4.21 


555 


18 


61 


4.85 






93 


66 


4.04 


485 


23 


65 


4.63 






94 


70.5 


3.89 


345 


29 


69 


4,44 


394 


25 


95 


75 


3.74 


275 


36 


73 


4,26 


310 


32 


96 


80 


3.59 


220 


45 


78 


4,09 


247 


44 


97 


85 


3.46 


179 


56 


83 


3.93 


195 


51 


98 


90 


8.33 


146 


69 


88 


3,78 


153 


65 


99 


95 


3.21 


121 


83 


94 


3.64 


126 


79 


100 


100 


3.10 


100 


100 


100 


3.5 


100 


100 


101 


106 


3,00 


82 


122 


106 


3.38 


84 


118 


102 


112 


2,91 


68 


147 


111 


3.27 


68 


146 


103 


118 


2,82 


56 


179 


116 


3.16 


58 


173 


104 


124 


2,73 


46 


217 


123 


3.05 


47 


211 


105 


130 


2.64 


38 


263 


129 


2.95 


39 


253 


106 


137 


2,56 


32 


313 


137 


2.85 


31 


316 


107 











143 


2.76 


26 


380 


108 





... 






152 


2.68 


21 


474 


109 










159 


2.60 


17 


575 


110 


--- 


... 


... 


... 


167 


2.53 


16 


637 



The resistance decreases rapidly as the temperature rises, and 
does not vary appreciably after the filament has passed red heat. 

Q. 518 — What efifect is produced by using a lamp on a circuit 
of different voltage from the voltage it was intended for ? 

A. — If the voltage is too low the candle-power is reduced, the 
efficiency reduced and the life is prolonged because the tempera- 
ture is below normal. If the voltage is too high, the opposite 
effects result. The accompanying table shows the results of vary- 
ing the voltage on 3.1 -watt General Electric lamps. 



CHAPTER XV* 
THE NERNST LAMP. 

Q. 519 — What is the third method of electric Hghting men- 
tioned under Q. 755? 

A. — The heating of a poor conductor to incandescence, some- 
what similarly to the manner of operating incandescent lamps. 

Q. 520 — What is the difference between the two methods ? 

A. — In the incandescent lamp the white-hot conductor is op- 
erated in a vacuum, as described. In the other lamp the illumi- 
nating conductor is enclosed in a globe, but the air is not ex- 
hausted; in fact, a little air is desirable, as in the case of the en- 
closed arc. 

Q. 521 — What is the conductor made of? 

A. — Refractory earths, such as magnesium, yttrium and. thor- 
ium, which have also been used in the Welsbach gas mantle. 
This form of lamp is known as the Nernst lamp, the utiliza- 
tion of rare earths in this manner having originated with Prof. 
Walther Nernst, a German scientist. 

Q. 522 — What is the form of the heated conductor? 

A. — It is made up in small sticks, one of the standard sizes 
being ^^o ii^ch in diameter and |i inch long. These are called 
"glowers." The glower of a Nernst lamp is a non-conductor when 
cold, and becomes a very poor conductor when hot. 

Q. 523 — If it is a non-conductor when cold, how does the cur- 
rent get through to heat it up ? 

A. — It doesn't. The glower must be warmed by some other 
means before it becomes sufficiently conductive to permit any cur- 
rent to pass. After current once starts through it, its tempera- 
ture is increased to white heat and maintained there by the cur- 
rent. 

Q. 524 — How is the glower heated ? 

A. — Usually by a coil of fine wire imbedded in cement for pro- 
tection from destruction, and located immediately adjacent to the 



202 



The Nernst Lamp, 



glower. The connections are as shown by Fig. 245. When the 
lamp is switched into circuit, current flows through the heating 
coil marked "heater," which immediately attains a high tempera- 
ture and heats the glower. As soon as enough current passes 
through the glower to raise its temperature, the magnet m attracts 
its armature and thereby breaks the heater circuit, leaving the 
glower in operation. 

Q. 525— What is the part marked bf 

A. — This is a resistance in series with the glower, and commonly 
known as the "ballast." 

Q. 526— What is it for ? 

A. — To prevent the glower from destroying itself by reducing 




I'ower.^.y. HEATER 

FIG. 245. 

its resistance sufliciently to pass an excessive current. The hotter 
the glower becomes the lower its resistance. 

Q. 527 — How does the ballast resistance prevent the glower 
from reducing its resistance too far ? 

A. — The ballast has a high positive temperature coefficient ; that 
is, its resistance increases rapidly with its temperature. Conse- 
quently a point is reached where an increase of current would in- 
crease the resistance of the ballast more than it would de- 
crease the resistance of the glower, and under this condition 
no increase in current can occur. 

Q. 528 — Does the glower waste away like the carbons of an 
arc lamp ? 

A. — No. The material is not subject to oxidation or combus- 
tion. On a direct-current circuit the glower deteriorates rapidly 
from electrolytic action, a black deposit being made on the nega- 



The Nernst Lamp. 



203 



tive end, which gradually extends clear across to the positive end 
and renders the glower worthless. 

Q. 529 — Does the glower deteriorate on alternating current ? 

A. — Yes, slowly; there is of course no electrolytic action, the 
deterioration being mechanical, due to the intense heat. 

Q. 530 — How are Nernst lamps connected to the circuit ? 

A. — In parallel, exactly like incandescents. 

Q. 531 — How much power do they require? 

A. — From i^ to 2 watts per candle-power. 




Q. 532 — Does the glower deteriorate in candle-power like the 
filament of an incandescent lamp ? 

A. — Yes, but not so rapidly. Most of the deterioration occurs 
just before the glower breaks. The lamp has not been in com- 
mercial service long enough at the time of writing to furnish 
reliable data as to candle-life. 

Q. 533 — In what form is the lamp made ? 

A. — Fig. 246 shows a single-glower lamp disassembled. They 
are also made with two, three, six and thirty glowers ; in multiple- 
glower lamps the glowers are connected in parallel. 



INDEX. 



Active Currents, q. 9, p. 1. 

Active E. M. F. q. 339, 340, p. 

140, 141. 
Adjustment of Brushes, q. 129, p. 38. 

Advantage of Compound-Wound Ma- 
chines, q. 199, p. 68. 

Advantages of Different Circuits for Mo- 
tors, q. 402, 408, p. 160 161. 

Alternating Current, q. 269, p. 108. 

Advantages of. q. 359-361, p. 148. 

Arc Lamps, q. 488, p. 190. 

Curve, q. 272, p. 109. 

Formulas, p. 147. 

Motors, Classes of. q. 409, p. 161. 

Reversals, q. 271-274, p. 109. 
Alternator, Inductor, q. 287-289, p. 115- 
118. 

Multipolar, q. 284-286, p. 113-115. 
Alternators, q. 283, p. 113. 

Types of. q. 284, p. 114. 
Amber, Attraction of. q. 2, p. 1. 
Ammeters, q. 257, p. 100. 

Amount of Current in Dynamo, q. 107, 

p. SO. 
Ampere, q. 26, p. 5. 
Standard, q. 27, p. 5. 
Turns, q. 72, p. 17. 
Value of. q. 27, p. 5. 
Analogy of Water, q. 14, p. 2. 
Angle of Lag. q. 328, 353-357, p. 134, 145, 

146. 
Apparent Watts, q. 352, 353, p. 144, 145. 
Arc, Normal Length of. q. 475, p. 186. 
Arc, Electric, with Alternating Current, 
q. 459-461, p. 182. 
Production of. q. 456-458, p. 181, 182. 

Voltage Required for. q. 459, 460, p. 

182. 

Arc, Enclosed, q. 462-464, p. 182. 

Voltage Required for. q. 467, p. 184. 
Arc Lamps, Alternating Current, q. 488, 
p. 199. 

In Series, q. 491, p. 191. 

Regulation of. q. 492, 493, p. 191, 192. 
Armature, q. 76, p. 18. 

Appearance of. q. 122, p. 35. 

"Drop" in. q. 200, p. 68. 
Armature Core, Construction of. q. 
118-120, p. 34. 

Drum. q. 104, p. 28. 

Dynamo, Attraction of. q. 103, p. 28. 

Ring. q. 104, p. 28. 

Winding of Four-Pole Machine. q. 
117, p. 33. 

Winding. Single-Phase, q. 290-293, p. 
119, 120. 



Armature Winding, Two-Phase, q. 294- 
298, p. 120-122. 
Winding, Three-Phase. q. 299-304, p. 

123, 124. 
Wires on. q. 105, p. 29. 
Attraction of Amber, q. 2, p. 1. 
Attraction of Dynamo Armature, q. 103, 

p. 28. 
Automatic Cut-Outs, Construction and 
Operation, q. 483-487, p. 189, 190. 

Ballast in Nernst Lamp, Action of. q. 
525-527, p. 202. 
Bar Magnets, q. 89, p. 23. 
Branch-Block, q. 218, p. 77. 
Branch Circuits, Use of. q. 227, p. 83. 
Brush Contact, Area of. q. 128, p. 38. 
Brushes, q. 100, p. 27. 

Carbon, q. 124, p. 36. 

Construction of. q. 124-128, p. 36-38. 

Copper, q. 124, p. 36. 

Position of. q. 129, p. 38. 
Bus-Bar. q. 191, p. 63. 

Calculation of Drop in Feeders, q. 207, 
p. 71. 
Carbon Brush, q. 124, p. 36. 
Carbons, Effect on, of D. C. and A. C. 
q. 470, p. 184. 
Feeding, q. 473-482, p. 184-188. 
Life of. q. 464, 466, 469, p. 182, 183, 

184. 
Shape and Size. q. 471, p. 184. 
Cell, Life of. q. 19, p. 3. 
Voltaic, q. 11, p. 2. 

Change of Field Connections by Con- 
troller, q. 179, p. 57. 

Circuit-Breakers, q. 213, 237, 239, p. 75, 
89, 90. 

Circuits, of Three-Phase Machines, q. 
305-310, p. 125, 126. 

Circular Mils. q. 207, 210, p. 71, 74. 

Clockwork Form of Clutch Mechanism. 

q. 477, p. 186. 
Coil, Adjustable Reactive, in Converter. 

q. 450, p. 177. 
Exciting, q. 289, p. 118. 
Coils, Induction, q. 289, p. 118. 
Collector Rings, q. 270, p. 109. 

For Three-Phase Winding, q. 301, p. 

123. 
For Two-Phase Winding, q. 295, p. 121. 
Combining Compound-Wound Dynamos. 

q. 197, 198, p. 66, 67. 

E. M. F.'s. q. 312-321, p. 127-131. 

E. M. F.'s Graphically, q. 316, p. 130. 

Series-Wound Dynamos, q. 196, p. 65. 
Commutated Field, q. 175, p. 56. 



206 



Index. 



Commutated Field Regulation, Use of. 
q. 176, p. 56. 

Commutating a Lagging Current, q. 329, 
p. 135. 

Commutator, q. 100, p. 27. 
Construction of. q. 123, p. 36. 

Compound-Wound Dynamo, q. 136, p. 43. 
In Parallel, q. 197, p. 66. 

Compound-Wound Machines, Advantages 

of. q. 199, p. 68. 
Concealed Wiring, q. 224, p. 80. 
Conductance, q. 263, p. 105. 
Conductivity, Electrical, of Copper Wire. 

q. 207, p. 72. 

Conductor, q. 14, 15, p. 3. 

Connecting Dynamos at Switchboard, q. 
244, 245, p. 92, 93. 

Connection between Car and Circuit, q. 
188, p. 61. 
Of Lamps in Cars. q. 190, p. 61. 
Of Rotary Converter, q. 445, 446, p. 
175. 
Constant Current Transformer, q. 498- 

503, p. 195, 196. 
Construction of Brushes, q. 124-128, p. 
36-38. 
Of Commutator, q. 123, p. 36. 
Contact Area of Commutator Brushes, q. 

128, p. 38. 
Controller, Operation of. q. 168-171, p. 

54-56. 
Converted Voltages in Converter, q. 449, 

p. 176. 
Converter, Converted Voltages in. q. 449, 
p. 176. 
Regulation of Voltage in. q. 449, p. 176. 
Converters, Reactive Regulators for. q. 

451-454, p. 177-180. 
Copper Brushes, q. 124, p. 36. 
Copper Wire, Conductivity of. q. 207, p. 
72. 

Core, Armature, Construction of. q. 118- 

120, p. 34. 
Coulomb, q. 28, p. 6. 
Counter E. M. F. q. 145, p. 46. 
"Crater" in Carbons, q. 461, p. 182. 
Current, Alternating, q. 269, p. 108 

Flow of. q. 18, p. 3. 

In Dynamos, Amount of. q. 107, p. 30. 

Lag of. q. 328, p. 134. 

Leak of. q. 16, p. 3. 

Rate of Flow of. q. 26, p. 5. 
Current Regulator for A. C. Arc Lights, 
q. 494-496, p. 192-194. 

Currents, Active, q. 9, p. 1. 

Eddy. q. 121, p. 35. 

Effect of. q. 21-24, p. 4. 
Curves, Alternating Current, p. 109, 122, 
123, 136. 

Cut-Out, Automatic, for Arc Lamps, q. 

483, p. 189. 
Cut-Outs. q. 217, 218, p. 76, 77. 
Cycle, q. 280-282, p. Ill, 112. 



r^ ecomposition of Liquids, q. 24, p. 4. 

Deflection of Needle, q. 22, 23, p. 4. 
"Delta" Connection, q. 311, p. 127. 
Density, Magnetic, q. 78, p. 20. 
Determination of Output of Transform- 
ers, q. 369, p. 150. 
Diagram of Battery Connections, p. 8-11. 

Of Brush Contact, p. 37. 

Of Combining E. M. F.'s. p. 130. 

Of Constant Potential Circuits, p. 69-71. 

Of Differential Field Winding of Mo- 
tor, p. 56. 

Of Dynamos in Parallel, p. 66, 67. 

Of Filament Deterioration, q. 514, p. 

199. 
Of Lamp Circuit, q. 202, 203, 205, 206, 

p. 69-71. 

Of Magnetic Circuit, p. 29, 31, 34. 

Of Motor Action, p. 45, 46. 

Of Rectifying Commutator, p. 133-135. 

Of Reversal of Motor, p. 57. 

Of Rheostats, p. 40, 41. 

Of Ring Armature, p. 28. 

Of Series-Parallel Control, p. 59. 

Of Three-Wire System, p. 64, 65. 

Of Wheatstone Bridge, p. 103-105. 

Diagrams of Alternating-Current Motor 
Connections, p. 165. 

Of Alternator Armature Windings, p. 
118-122. 

Of Arc-Lamp Circuits, p. 188, 189, 191, 
192. 

Of Car Circuits, p. 62, 63. 

Of Converter Connections, p. 174, 180. 

Of Dynamo Windings, p. 39, 42-44. 

Of Field Regulation of Motor, p. 53-55. 

Of Fuse Layouts, p. 74, 75. 

Of Impedance, p. 143, 144. 

Of Switchboard Connections, p. 90-93. 

Of Three-Phase Circuits, p. 125-128. 

Of Transformer Windings and Connec- 
tions, p. 152-158. 
Differential Clutch Mechanism for Arc 

Lamps, q. 473, p. 185. 
Diflferential Field Winding, q. 174, p. 56. 
Discharge of Electricity, q. 8, p. 1. 
"Drop" in Armature, q. 200, p. 68. 

In Circuit, q. 200, 201, 204, 207, p. 68, 
69, 71. 
Dynamo, Compound- Wound, q. 101, p. 27. 

Regulation of Output, q. 133, p. 40. 

Series- Wound, q. 130, p. 39. 

Shunt-Wound, q. 130, p. 39. 
Drum Armature, q. 104, p. 28. 
Dynamo Armature, q. 101, p. 27. 
Dynamos, Multipolar, q. 115, p. 32. 

In Parallel, q. 191, p. 62. 

In Series, q. 192, p. 64. 

Pddy Currents, q. 121, p. 35. 

Effect of Resistance on Speed of 
Motor, q. 147, p. 47. 



Index. 



207 



Effect of Series Coils, q. 137, p. 43. 

Of Winding on Capacity of Wires, q. 
113, p. 31. 

Effective E. M. F. q. 276-279, p. 110, 111. 

Effects of Currents, q. 21-24, p. 4. 

Efficiency of Incandescent Lamp. q. 510, 
p. 198. 

Of Solenoid, q. 87, p. 22. 

Electric Arc with Alternating Current, 
q. 459-461, p. 182. 

Production of. q. 456-458, p. 181, 182. 

Voltage Required for. q. 459, 460, p. 
182. 

Electric Light, Methods of Production 
of. q. 455, p. 181. 

Production of, by Carbon Vapor, q. 456, 
p. 181. 

Electric Pressure, q. 39, p. 7. 

Electrified Bodies, q. 8, p. 1. 

Electrodes, q. 17, p. 3. 

Electro-Magnet, Definition of. q. 69, p. 16. 

Electro-Magnetic Induction, q. 94, p. 25. 

How Obtained, q. 95, p. 25. 
E. M. F. q. 39, p. 7. 

Active, q. 339, 340, p. 140, 141. 

Generation of, by Moving Conductor, 
q. 95, p. 25. 

Impressed, q. 337, 338, p. 140. 

Inductive, q. 335, p. 139. 

Secondary, q. 366, p. 150. 

Enclosed Arc Lamp. q. 462, p. 182. 

Equalizer Bus-Bar. q. 197, p. 66. 

Excitation of Alternator Field Magnets, 
q. 322, p. 132. 

Exciter, for Synchronous Motor, q. 412- 
414, p. 162. 

Exciting Coil. q. 289, p. 118. 



Ceeder. q. 202, p. 69. 

Field Connections, Change of, by 
Controller, q. 179, p. 57. 

Field Magnet, Revolving. 

Field Magnets, Alternator, Excitation of. 
q. 322, p. 132. 

Field Winding, Use of Different, q. 134, 
135, p. 41. 

Filament of Incandescent Lamp. q. 504, 

p. 197. 

First Mention of Electricity, q. 1, p. 1. 

Flow of Current, q. 17, 18, p. 3. 

Foot-Pound, Relation to Joules, q. 59, 
p. 13. 

Foot-Pounds. q. 152, p. 48. 

Formula for E. M. F. Generated in Dy- 
namo, q. 106, p. 30. 
Foucault Currents, see Eddy Currents. 
Frequency, q. 281, p. 111. 
Frictional Electricity, q. 4, p. 1. 
Fuse, Size of. q. 215, p. 75. 
Fuse Blocks, q. 217, p. 76. 
Fuses, q. 213, p. 75. 
In a Series Circuit, q. 219, p. 78. 



/galvanometer, q. 261, p. 102. 

Generation of Current by Moving 
Conductor, q. 97-100, p. 26, 27. 

Generation of E. M. F. by Moving Con- 
ductor, q. 95, p. 25. 

Glower, Heating of, in Nernst Lamp. q. 

524, p. 201. 
Glowers in Nernst Lamp. q. 522, p. 201. 
Graphical Combination of E. M. F.'s. q. 

316, 317, p. 130. 

Heating of Armature Wires, q. 108-112, 
p. 30, 31. 
Of Wires, q. 211, p. 74. 
Horse-Power, Electrical, q. 57, p. 12. 

Of Motors, q. 153-155, p. 49. 
Horseshoe Magnets, q. 89, p. 23. 



Impedance, q. 342-350, p. 141-144. 

Impedance Diagrams, q. 347, p. 143. 

Impressed E. M. F. q. 337, 338, p. 140. 

Incandescent Lamp, Amount of Current 
Necessary, q. 506, p. 198. 

Constntction of. q. 504, p. 197. 
Effect of Abnormal Voltage, q. 518, p. 
200. 

Efficiency of. q. 510, p. 198. 
Evaporation of Filament, q. 513, p. 198. 
Filament Deterioration, q. 514, 515, p. 

199. 
Life of. q. 509, p. 198. 
Production of Light in. q. 5C^, p. 197. 
Resistance of Filament, q. 517, p. 200. 
Temperature of Filament, q. 512, p. 198. 

Use of Platinum Connections in. q. 
507, p. 198. 

Standard Efficiencies, q. 516-, p. 200. 

Inductance, q. 336, p. 139. 

Induction, q. 94, p. 25. 

Induction Coils, q. 289, p. 118. 

Induction Motor, q. 410, p. 161. 

Advantage of Three-Phase. q. 436, p. 
170. 

Arrangement of Field Coils, q. 425, p. 

166. 
Construction of. q. 424, p. 165. 
Current in Armature, q. 430, p. 168. 
Difference between Two and Three 

Phase, q. 435, p. 170. 
Field Windings, q. 426, p. 167. 

Speed of Rotor, q. 431-433, 437, p. 168, 
169, 171. 

Torque of Rotor, q. 432, p. 169. 
Inductive Action, q. 332, p. 137. 
Inductive E. M. F. q. 335, p. 139. 
Inductor, q. 289, p. 118. 
Insulators, q. 15, 16, p. 3. 

Joule, q. 58, p. 13. 

Relation to Foot-Pound. q. 59, p. IS. 
Junction Box. q. 227, p. 83. 



208 



Index. 



l^ilowatt. q. 60, p. 13. 

Kilowatt-Hour. q. 60, p. 13. 



I ag of Current, q. 328, p. 134. 

Lamination of Round Rod. q. 490, 
p. 190. 
Lamp Circuit, Diagram of. q. 202, 203, 

p. 69. 
Lamp Circuits in Cars. q. 190, p. 61. 
Lamps, Enclosed Arc. q. 462, p. 182. 
Law of Magnetic Circuit, q. 78, p. 20. 
Leak of Current, q. 16, p. 3. 
Life of a Cell. q. 19, p. 3. 
Life of Incandescent Lamp. q. 509, p. 198. 

Light, Production of, by Electricity, q. 

455, p. 181. 
Lightning Arresters, q. 250-255, p. 94-98. 
Liquids, Decomposition of. q. 24, p. 4. 
Load on Motor, Determination of 

Amount, q. 151, 152, p. 48. 

Loss in Armature Circuit, q. 138, p. 44. 
Loss of Energy in Brushes, q. 126, p. 37. 



TWf agnet. Definition of. q. 63, p. 15. 

Polarity of. q. 70, 71, p. 16. 

Strength of. q. 72, p. 17. 

Magnetic Attraction, Cause of. q. 74, 
p. 18. 

Magnetic Circuit, Law of. q. 78, p. 20. 
Magnetic Density, q. 78, p. 20. 
Magnetic Field, Definition of. q. 65, p. 15. 
Magnetic Flux, Definition of. q. 65, p. 15. 
Magnetic Flux in Transformer, Determi- 
nation of. q. 371-373, p. 151. 
Magnetic Lines, q. 64, p. 15. 
Magnetic Quality, q. 72, 73, p. 17. 

Magnetic Repulsion in Constant Current 
Transformer, q. 500, p. 195. 

Magnetic Resistance, q. 77, p. 19. 

Magnetic Saturation, q. 77, p. 19. 

Magnetism, Definition of. q. 64, p. 15. 

Residual, in Field Coils, q. 132, p. 40. 
Magnets, Bar. q. 89, p. 23. 

Horseshoe, q. 89, p. 23. 

Permanent, How Made. q. 92, p. 24. 

Permanent, Material for. q. 93, p. 24. 

Use of. q. 75, p. 18. 
Mains, q. 202, p. 69. 
Mathematical Relations of Alternating 

Current Values, p. 147. 
Maximum Safe Current, q. 211, p. 74. 
Mil, q. 475, p. 186. 
Molding Work. q. 223, p. 80. 
Motor, q. 139, p. 44. 

Motor, Diflferential Field Winding for. 
q. 172, p. 56. 

Induction, q. 410, p. 161. 
Induction, Construction of. q. 424, p. 
165. 



Motor, Load On, Determination of 
Amount, q. 151, 152, p. 48. 

Reversal of. q. 178, p. 57. 

Starting, q. 160-167, p. 51-53. 

Synchronous, q. 410, p. 161. 
Motor Connections, q. 160, p. 51. 

Railway, q. 160, p. 51. 

Motors, Arrangement for Field Regula- 
tion, q. 168, 169, p. 53, 54. 

Compound- Wound, q. 172, 173, p. 56. 

Compound Wound, for Variable 
Speeds, q. 185, p. 60. 

Regulation of Speed of. q. 156, 159, 168, 

171, 180, 182, p. 49, 50, 53, 56, 59, 60. 

Series-Wound, Advantage of. q. 184, 
p. 60. 

Multiple Connection, q. 48, 49, p. 9. 

Multipolar Dynamos, q. 115, p. 32. 



Nernst Lamp, Construction of. q. 519- 
522, p. 201. 

Nernst Lamps, Connection of. q. 530, 
p. 203. 

Deterioration of. q. 532, p. 203. 

Form of. q. 533, p. 203. 

Heater in. q. 524, p. 202. 

Life of Glower, q. 528, 529, p. 202, 203. 

Operation of. q. 523-533, p. 201-203. 

Power Required for. q. 531, p. 203. 
Neutral Points, q. 129, p. 38. 
Neutral Wire. q. 192, p. 64. 



f\\im, Definition of. q. 34, p. 6. 
^ Ohm's Law. q. 40, p. 7. 
Operation of Rotary Converter, q. 442- 
444, p. 173, 174. 



parallel Connection, q. 48, p. 9. 

Parallel-Series, q. 50-55, p. 10. 
Paths of Current through Armature, q. 

113, 114, p. 32. 
Period, q. 281, p. 111. 

Permanent Magnets, Use of. q. 91,. p. 23. 
Permeability, q. 77, p. 20. 

Table of. q. 77, p. 19. 
Phase Relations, q. 349, p. 144. 
Polarity, Determination of. q. 71, p. 16. 
Polarity of Magnet, q. 70, 71, p. 16. 
Poles of Magnet, q. 68, p. 16. 
Polyphase Distribution, Advantages of. 

q. 402, p. 160. 
Potential Regulator for A. C. Arc 

Lights, q. 497, p. 194, 195. 
Pounds, Foot. q. 152, p. 48. 
Power Factor, q. 357, 358, p. 146. 
Primary Winding in Transformers, q. 

363, p. 149. 



Index. 



209 



Ratios of Transformation in Transform- 
ers, q. 365-368, p. 150. 
Ratios of Windings in Transformers, q. 

368, p. 150. 
Reactance, q. 342-350, p. 141-144. 
In Arc-Lamp Circuits, q. 492, 493, p. 
191, 192. 
Reactive Regulators for Converters, q. 
451-454, p. 177-180. 

Rectifying Commutator. q. 323-327, p. 

133, 134. 
Regulating Output of Dynamo, q. 133, 

p. 40. 
Regulator for Alternating Current Arc 

Lamps, q. 494, p. 192. 
Relation of Units, q. 62, p. 13. 
Reluctance, q. 78, 287, p. 20, 116. 
Residual Magnetism in Field Coils, q. 

132, p. 40. 

Resistance, q. 263, p. 105. 

Definition of. q. 29, p. 6. 

Properties Affecting, q. 31, 33, p. 6. 
Resistance Measured by "Drop." q. 268, 
p. 107. 

Of Parallel Circuits, q. 264, p. 106. 
Reversal of Motor, q. 178, p. 57. 
Rheostat for Starting Motor, q. 160-167, 

p. 51-53. 
Ring Armature, q. 104, p. 28. 
Rise in Temperature, Computation of. 

q. 112, p. 31. 
Rocker-Arm. q. 129, p. 38. 
Rotary Converter, q. 440, p. 173. 

Connections, q. 445, 446, p. 175. 

Operation of. q. 442-444, p. 173, 174. 

Speed of. q. 447, p. 176. 

Use of. q. 441, p. 173. 
Rotary Field, q. 427, p. 168. 
Rotor, q. 289, p. 118. 
Rotor Windings, q. 429, p. 168. 
Rules for Connections, q. 54, p. 11. 



Saturation, Magnetic, q. 77, p. 19. 

Secondary E. M. F. q. 366, p. 150. 
Secondary W'inding in Transformers, q. 

363, p. 149. 
Self-induction, q. 331-334, p. 137-139. 
Series Connection, q. 45, p. 8. 

Series-Parallel Connection of Batteries. 

q. 50-55, p. 10. 
Series-Parallel System, q. 192, p. 64. 
Series-Wound Dynamo, q. 130, p. 39. 

Series-Wound Dynamos in Series, q. 
196, p. 65. 

Series-Wound Motors, Advantage of. q. 
184, p. 60. 

"Short-Circuit." q. 212, p. 75. 

Shunt, A. q. 131, p. 39. 

Shunt Lamp Mechanism, q. 477-481, p. 
187, 188. 

Shunt-Wound Dynamo, q. 130, p. 39. 
Single-Phase Armature Winding, q. 290- 
293, p. 119, 120. 



Size of Wires in Three-Wire System, q. 
209, p. 73. 

Sizes and Capacities of Wire. q. 207, 

p. 72. 
Solenoid, q. 82, p. 21. 

Efficiency of. q. 87, p. 22. 
Sparking, q. 125, p. 36. 
Speed of Motor, q. 146, p. 47. 

Speed of Motors, Determination of. q. 
149, 150, p. 47, 48. 

Effect of Resistance on. q. 147, p. 47. 

Regulation of. q. 156, 159, 168, 171, 180, 
182, p. 49, 50, 53, 56, 59, 60. 
Speed of Rotary Converter, q. 447, p. 176. 
Standard Ampere, q. 27, p. 5. 
Standard Ohm. q. 34, p. 6. 
Standard Volt. q. 38, p. 7. 
"Star" Connection, q. 311, p. 127. 
Static Electricity, q. 4, p. 1. 
Stator. q. 289, p. 118. 
Stator Windings, q. 426, p. 167. 
Steel, Tungsten, Analysis of. q, 93, p. 24. 
Step-Up Transformer, q. 396, 397, p. 157. 
Strength of Magnet, q. 72, p. 17. 
Switch, q. 228, p. 83. 
Switch, Double-Break, q. 230, p. 84. 

Double-Pole. q. 233, p. 85. 

Double-Throw, q. 235, p. 86. 

Single-Break, q. 230, p. 83. 
. Single-Pole Button, q. 230, p. 83. 

Single-Pole Knife, q. 230, p. 83. 

Three-Pole. q. 233, p. 80. 
Switchboard, q. 236, p. 88. 

Central Station, q. 246-249, p. 93, 94. 

Switchboard Connections, q. 239-242, p. 
90, 91. 

Synchronous Motor, q. 410, p. 161. 

Soeed of. q. 415, 416, 420, 421, p. 162, 
163, 164. 

Starting a. q. 417-419, p. 163, 164. 

Synchronous Motors, Difference between 
Single and Polyphase, q. 422, p. 165. 



'Tablet Board, q. 226, p. 82. 

Taps. q. 202, p. 69. 

Temperature, Allowable Rise. q. 378, p. 
152. 

Theory of Motor Operation, q. 140-145, 
p. 44-46. 

Three Phase Armature Winding, q. 299- 
304, p. 123, 124. 

Three-Wire System, q. 192-195, p. 64, 65. 

Size of Wires, q. 209, p. 73. 
Torque, Formula for. q. 154, p. 49. 
Torque of Motor Armature, q. 152, p. 48. 
Transformer, q. 362, p. 148. 

Allowable Current in. q. 375, p. 151. 

Constant Current, q. 498-503, p. 195, 
196. 

Construction of. q. 363, p. 149. 

Current in Outside Circuit, q. 379, p. 
152. 



2IO 



Index 



Transformer, Determination of Magnetic 
Flux in Core of. q. 371, p. 151. 

Operation of. q. 364, p. 149. 

Radiating Surface in. q. 376, p. 152. 

Step-Up. q. 396, 397, p. 157. 

Windings and Connections, q. 380-394, 
p. 152-156. 

Transformer Secondaries, Connected for 
Three-Wire Circuit, q. 390, p. 155. 

Series and Parallel Connections, q. 388, 
p. 154. 
Transformers, on Polyphase Circuits, q. 
400, 401, p. 158, 159. 
Ratios of Current in. q. 374, p. 151. 
Ratios of E. M. F.'s in. q. 368, p. 150. 
Ratios of Windings in. q. 368, p. 150. 
Transmission of Power to Car Axle. q. 

186, p. 61. 
Two-Phase Armature Winding, q. 294- 
298, p. 120-122. 



I T nits. Relation of. q. 62, p. 13. 

*-^ Use of Commutator in Motor. q. 

144, p. 46. 

Use of Different Field Windings, q. 134, 
p. 41. 

Use of Magnets, q. 75, p. 18. 



yolt, Definition of. q. 36, p. 7. 

Standard, q. 38, p. 7. 
Voltage, q. 39, p. 7. 
Voltaic Cell. q. 11, p. 2. 

Voltmeter Switch Connections, q. 243, 

p. 91. 
Voltmeters, q. 256, p. 99. 

Y^ ater, Analogy of. q. 14, p. 2. 

Watt, Definition of. q. 56, p. 12. 
Watt-Hour. q. 60, p. 13. 
Wattmeters, q. 258-260, p. 101, 102. 
Watts, Apparent, q. 352, 353, p. 144, 145. 
Watts, True. q. 353, p. 145. 
Wheatstone Bridge, q. 262-267, p. 103-106. 
Wire, Size of Necessary, q. 377, p. 152. 
Wire Sizes and Capacities, q. 207, p. 72. 
Wires on Armature, q. 105, p. 29. 
Wiring, Classes of. q. 221, p. 78. 

Cleat, q. 222, p. 79. 

Concealed, q. 224, p. 80. 

Conduit, q. 224, p. 81. 
Wiring System, Choice of. q. 225, p. 81. 

; V " Connection, q. 311, p. 127. 




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